I totally agree with this:
"I'm quite intrigued by this as actually as I had sort of assumed that even if a guy prefered to bottom it wouldn't normally be so hard to persuade him to top. More maybe than getting a top to bottom."
Whereas I do both and enjoy them equally (if differently), I can see why some guys wouldn't take to bottoming. However, out of every 10 "total bottoms" I meet, a surprising number of them will not top under any circumstance. When I am in bottom mode, it really doesn't take a whole lot of persuading to get me to rise to the occasion and top my partner.
I also think it has a lot to do with the type of guy you are attracted to. I tend to look at a guy and let my cock tell me if I want to top or bottom. Not exactly sure what it is, but when I was younger I was much more of a bottom. I was typically attracted to older, more masculine guys. Possibly lack of experience or expertise led me to believe that being a bottom was being "passive" partner. In fact, I would sometimes joke that I was a bottom simply because I was kind of lazy. How wrong I was!
Nowadays, I'm attracted to guys closer to my age and I see myself as being much more versatile than ever before.