I've been both a top and a vers in my sexual career and I can give you answers to the why for both reasons.
When I was younger I was VERY curious about bottoming, but even more so about topping. When I did lose my virginity to a man (age 20) it was made abundantly clear to me by the guy I was with that I definitely had a big dick, and he explained to me I could expect a lot of attention in my life because of that. Now, I knew I was big before, but that kind of stuck with me, and suddenly it was lick a switch. I then was only interested in being a top.
Fast forward about a decade and I found myself single. Now I HAD bottomed (maybe twice) for my ex at this point, but it had lost its appeal. In retrospect it was all in the why / how I had been topped. Bottoming can be a vulnerable position, both emotionally and physically, and it hadn't gone well for me and I had lost my taste. However, in my early 30's I briefly dated a guy who was only a top. I didn't know that then. He did top me, and it was quite a positive experience, but nothing came of the relationship.
Fast forward and a failed marriage later, I found myself single, and dating. Never really thought about bottoming at all. Then I met my future husband. He presented as an absolute bottom, and hey, that fit my top card just fine.
Then one day several months into our burgeoning relationship and I found myself sucking him, and he very pointedly told me that if I kept it up, he'd top me. Now
@Faunus is not small and I admit I was turned on by that thought. About a day later he topped me, and it was absolutely wonderful.
Since then I'm pretty firmly in the vers camp - with an emphasis on the top part of vers.
Why?
Because I've found that I do enjoy bottoming. However - caveat - is that it is (so far) only with him that I've bottomed. For me its a comfort level. I do want to explore my bottomhood a bit more - but so far I have not found anyone I'm comfortable with in that regard, because the husband has more than enough dick for me, and not many come close to us in size (at least in person). So - there is that as well.
TL

R - Top or bottom, its because I enjoy doing it - both the feeling of being a top and being a bottom and the differences they bring to my sex life. Variety is fun!