As you point out, for "the kind of editing you do" Vegas is a great tool.
I think the kind of editing someone does and their workflow is extremely important.
Workflow on a highend movie usually involves a series of specialists
an assistant editor to put together a rough assembly of a scene (possibly one per scene)
the editor who puts the final assembly together
a colorist who does the color correction
a sound editor who does the sound editing (Also probably with Assistants)
a composer who does the score.
someone who does automatic dialogue replacement
probably some others that I forgot.
So you need a workflow that allows you to shift the media and the project to all the above people and the various software they use to do the above. Often you also need a way of taking your edit back from you NLE to film. Remember the All-digital workflow is actually still a very new thing. Playing well with others and going back out to film is what Avid and FCP do well. Vegas is designed with a philosophy of "we can do it all in Vegas."
Another thing involved with high-end film is media-management becomes extremely important. When you got 42 takes of a shot, you want to be able to get to the right ones quickly. This is one area where Avid editors have a problem with Vegas.
HIGH end codecs. Before Vegas 8, it only could use 8 bit codecs, I believe. Vegas 8 introduced 32-bit color processing, but it was a bit clunky to use.
Most Important: Tons and tons of trained Professional Avid and FCP editors. One-man-band editors LOVE Vegas, however, if you are opening a production house that edits 10 reality TV shows and you need 20 well trained editors in your market, you're more likely to go with Avid or FCP. College Students studying film are far more likely to be trained in Avid and FCP.
Once a market is entrenched in Avid or FCP, you really have to create a great tool to dislodge people. They are trained and proficient in a specific workflow. They have a large investment in the tools. If they are pros who have a great amount of work, when do they find the time to learn a new way. If you are coming completely new to editing, it's much, much easier just to pick the best tool for you and for a lot of folks that's Vegas.