There are a couple of reasons:
1. When you find two brands in a shop, it's usually a major player like Scubapro or Aqualung, along with some other brand they can heavily discount. But you generally don't find Scubapro and Aqualung together as often as you did in the past. Way way back in the day, Aqualung/US Divers was a powerhouse in the dive industry, before Scubapro even got rolling in 1963.
By the 1970s Scubapro was well regarded, however in the 1970s and 80s Aqualung/US Divers' quality slumped a lot and they became associated with main order sales and were not regarded as a tier one company. That's the era where you found Scubapro and Aqualung in the same shop - the shop would carry Scubapro for higher end sales and US Divers for mid level and entry level sales to divers on a budget.
Aqualung rebounded from that and is once again a tier 1 brand, and it's difficult to support two tier one brands in a single shop, unless it's a large shop as you can't generate enough sales of both to keep both companies happy.
2. Scubapro is also a "piston regulator" company. They made their name on bullet proof unbalanced and balanced piston designs and they can't seem to get past that. For example, their Mk 17 is arguably the best diaphragm first stage ever made, but Scubapro has always given it second billing to the Mk 25 - a regular that has plenty of flaws in extremely cold water.
Old school Scubapro shops have drank gallons of the piston kool-aid overt the years and have a strong and long standing bias against diaphragm regs. Those shops are not real happy carrying the Mk 11 and Mk 17, and they wouldn't dream of carrying a brand like Aqualung that specializes in diaphragm first stages.
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Both companies build excellent regulators and just as importantly both companies offer superb long term support, including support for most discontinued models.
Personally, if Scubapro ever stops making the Mk 17 and I need to replace the ones I have, I'd buy an Aqualung Legend LX in a heartbeat. It's the next best thing to a Mk 17, and it's a very well made first stage. The second stage is for all intents and purposes a G250 knock off in terms of the poppet (like the old Scubapro blue plastic and metal poppet, but in a lighter all plastic form, without the streamlining of the current Scubapro S-Wing poppet) and overall design.
