I think for many it's a lack of exposure during training to deal with the complex issues or complex medical needs that many individuals with disabilities have. For one thing, there's a lack of comfort level, which may deter them from realizing they can, with practice or mentorship, work on that. It's not really structured into education or post-graduate training unless you're in a specialty that deals with it.
There could also be some physical barriers with equipment, such as specialized chairs that can accommodate wheelchairs, or lack of access to hospital service, where a lot of these individuals need to be. Lots of hospitals or dental departments within hospitals have had cuts over time, so it's very hard to get them there.