Well, price was definitely the first factor for me. I also wanted a Solitaire in landscape mode.
Solitaire Top 3 was my first choice since I only knew how to play Klondike and Spider (and the icon was decent as well as I figured the name would simply be "Solitaire" on the home screen). After seeing how it looked in portrait, I didn't really like it.
mondo was my next purchase (not my first due to the reviews of lagginess) because of the aesthetics. It just seemed like $10 for the all the little details like the magnifying glass and the paperclip (found out I didn't really like either) I read a review saying the sluggishness wasn't that much. Turns out I was wrong.
Platinum Solitaire was nice, but it was a bit slow (not in animation, just picking up the cards).
Solitaire Forever I played on the Mac and it was kind of okay (not good to keep all the animations and stuff on a small screen though) and I found portrait mode impossible to play with due to the fact that they angle the stage.
I think the two things that made me not buy Solitaire City was the only portrait mode, the fact that mondo looks a lot better which was the standard for the $9.99 price, and the icon. The background also made me stop (I thought there was a set background for every game and the sunset screenshot looked kind of ugly) It definitely runs amazingly smooth and it definitely looks the best. Wish that I bought it first and stuck with it. I guess I assumed that landscape wasn't coming. But the stats and the wide variety of games is definitely a plus.
I bought Solebon after Solitaire City because of the icon (until I found out Solebon is tap only).
Of course being a programmer doesn't mean a good graphics artist, but I'd definitely be set once the icon looks simple and appealing (it's just me. I'm a graphics artist and I like things to look nice

)
As for other people, I believe it's mainly because of the price. With Solitaire Top 3 actually looking nice in landscape and with most people only knowing Klondike, Freecell, and Spider, most people go for that first.
But after using the surprise setting in mondo, I found that I've come to like a lot of variations (then would go to Solitaire City to find that they're there and play them

) I guess there's not much you need to update. Everything's pretty much perfect in Solitaire City. Maybe more games but that's about it.