I teach technical wreck, not cave, (although the syllabus is largely identical)... and those courses have the benefit that the student is supposed to already be a competent and experienced technical diver before applying those skills and equipment in the overhead.
In cave, there can be a lot less 'equipment time' before you enter the overhead environment.
In general, I find opposition to sidemount comes from only four sources:
1) Divers who've been badly trained on sidemount, and consequently have not enjoyed the performance and comfort benefits that sidemount should provide.
2) Divers who've encountered badly trained sidemount divers and attribute the shenanigans they've witnessed as being a result of an equipment performance deficit, not a training deficit.
3) instructors who aren't effective sidemount divers themselves, and seek to conceal the competency shortfall by downplaying the performance of sidemount equipment.
4) Experienced and highly competent (esp tech/cave) backmount divers who 'try' sidemount and obviously notice a performance degradation due to the unfamiliar kit and protocols. Unwilling to be a 'learner' again, they instead deny the need to build a comparable experience foundation in that unfamiliar kit. They dismiss sidemount as "more complex", "not as useful" or "not as natural" etc etc etc... Basically, anything other than simply accepting that you can't compare your performance, fluidity and comfort in kit you've used thousands of times, versus kit you've barely used at all.
The AVAILABILITY (of quality tuition) is much more in favor of backmount...
An attitude that emphasises that they don't appreciate the need (when it doesn't suit them) to spend time in a given configuration to develop a deeply ingrained, intuitive and unconscious level equipment operation and protocols familiarity...
Which leads to situation #4, that I mentioned above...
In short, egos can't easily handle not still being the 'expert' in that equipment, so denial and avoidance are preferable courses of action
