As joe8mofo states, there are other criteria which contribute to most agencies setting 130ft as a maximum depth limit for recreational divers. Narcosis management is prime among them - and has an impact on all the other criteria.
Without technical training, recreational divers are not trained, by default, in certain key deep diving competencies. Neither are they expected, or required, to have a minimum level of redundancy (beyond the existence of a buddy).
Narcosis
Nitrogen/Inert Gas Narcosis typically starts to degrade performance around 30m/100ft. The effects become much stronger for each few meters beyond that. For many divers, especially those without ingrained functional diving skills, they will become debilitating or at the least, significantly reduce performance, beyond 40m/130ft. Symptoms of narcosis may not be present, but a diver will still be impaired. The typical CO2 retention associated with stressful incidents (exertion and/or elevated respiration) can also cause a rapid spike in narcosis and catastrophically degrade mental performance when it is most needed to save your life.
Time to Surface
If you consider 40m/130ft in respect of 'time-to-the-surface' then (depending on your training agency) you have 2-4 minutes of ascent travel to reach safety should a problem arise. That is a long time to potentially have no air to breath - and beyond the capabilities of many people. In contrast, 18m/60ft (1-2 minute ascent) is easily within the capabilities of most people to achieve on a single breath/exhalation. In essence, the 'last fail-safe' of a Controlled Emergency Swimming Ascent can be denied to the diver.
Gas Consumption
Whilst your respiration may remain constant, there is a direct relationship between the volume of gas consumed and the ambient pressure it is breathed at. Deeper diving will entail highly elevated consumption of gas supplies, requiring greater situational awareness and more diligent monitoring of gauges. The duration of deep dives will be significantly limited by the gas supply available to the diver. Recreational divers rarely receive the training necessary to calculate their air consumption at a given depth and for the following ascent.
Equipment Functionality
The effects of increased ambient pressure and workload dealing with denser gas flow causes more strain on scuba equipment. This can lead to increased risk of equipment failure. When coupled with the other factors listed, an equipment failure below 40m/130ft is significantly more serious that it would be within recreational diving depths. Whilst technical divers are trained to identify such risks and mitigate them through a calculated system of equipment/gas redundancy and refined emergency procedures, there is little provision of these safeguards within recreational diving course syllabus.
Bottom Time
It is possible to calculate dives beyond 40m/130ft that still provide an NDL. Even the PADI RDP states a 4 minute bottom time at that maximum depth. Whether such times, especially when descent time is considered, remain worthwhile or meaningful is very questionable. At the most, the diver will be restricted to a '
bounce dive' - which presents further complications and risks from a physiological/decompression perspective. Also, as per gas consumption, the shorter duration of bottom demands a much higher degree of situational awareness and 'gauge vigilance' from the diver. A distraction, or any unforeseen delay, can much more easily lead to the imposition of mandatory (emergency) decompression stops, which a recreational diver is unlikely to be trained or equipped to complete with any guarantee of effectiveness.
Decompression Sickness
Decompression software, used to calculate dive times via tables or dive computers, is not an exact science. At best, it can be considered in respect of 'statistical likelihood'. Adhering to a No-Decompression Limit does not ensure invulnerability to DCS. On a given day, a given diver will be subjected to a greater, or lesser, degree of pre-cursor factors towards DCS. Likewise, NDLs can only be considered relative to the ascent that follows them. Failure to ascend at the correct speed, or perform precautionary 'safety stops' further increase risk. Divers do get bent within NDLs. Needless to say, all agencies encourage divers to plan dives conservatively; allowing a healthy buffer against their NDL. Diving below 40m/130ft virtually guarantees that no meaningful buffer can exist. You can get 'unlucky' quicker... and there is virtually zero tolerance for any mistakes.
Oxygen Toxicity
Exposure to high oxygen partial pressure (PPO2) entails the risk of oxygen toxicity (convulsions, typically leading to drowning). A healthy buffer exists between the recreational diving limit of 40m/130ft and the depth (~56m) where breathing air exceeds the advised maximum PPO2 of 1.4. However, as with DCS, risk of incident is nothing more than statistical likelihood. You are not guaranteed immune from O2 toxicity below PPO2 1.4 - other predisposing physiological factors can have an impact on susceptibility. Few recreational divers are educated to understand those predisposing factors, or calculate a prudent max PPO2 in relation to the nature of the dive they are undertaking.
CO2
Carbon Dioxide plays a key role in many of the factors already listed. The retention of CO2 is a contributing factor to narcosis and oxygen toxicity risk. CO2 levels in the lungs are fundamental to respiration control - the degradation of which is a major catalyst to stress and uncontrolled panic. CO2 retention is increasingly likely as depth increases, due to the density of gas breathed and consequent workload on the lungs. This especially true when combined with exertion, narcosis symptoms, underlying stress and/or poor regulator performance.
In my opinion, any one of these factors is sufficient reason to not venture below 40m/130ft, without undertaking the technical level training necessary to mitigate the consequent risks. When considering the relationship between these factors - and the high likelihood that multiple issues will manifest together, one being a result of the other - along with understand of the limits of training and capability provided within the recreational diving syllabus...an extremely strong basis is created for the imposition of a 40m/130ft maximum depth for recreational divers.
