One reason is that the Linux version of Flash is terrible last time I used it. Not even from a "open web principles" or "proprietary software is evil!" point of view, but rather in a "it crashes the browser on every 2nd or 3rd video viewing" perspective.
Also, I think Flash on OSX buffers more slowly than on Windows. It's why whenever I want to see something online I have to reboot into my Windows partition, where the video almost never has to stop to buffer. On OSX however, buffering for several minutes only leads to about 20-30 seconds of the actual video being loaded.
I don't get that, at least on 32bit Ubuntu at home, although I hear people have a pretty crap experience. Have always found it pretty stable on Fedora / Ubuntu for the last decade actually - I get the feeling they do all their testing on major distros only.
However we've just moved to 64bit Linux Flash 10 alpha on RHEL at work - which I suspect might change my impression. It has Z axis issues at least which the current 32bit one doesn't.
Run it in nspluginwrapper, so that when Flash crashes it doesn't take down the browser. You just have to refresh the page for the Flash object to run again. Flash crashes less than once a day for me, and I use Flash a lot.