Payouts are the main reason, I do have alot of sites (and galleries) there, and income on ARS has always been larger than on other programs (not just by proportion, comparing # of my sites there and on other programs), I get alot more $ PER SITE in the long run. You also get some fair traffic, and the whole system is pretty smooth & well thought out (not a simple plugin/addon).. Oh and the staff is very friendly/quick.
I heared that some ppl had good results with pimproll, for some reason I got my tour approved there but never added to the system, their messaging system went belly-up for a short time (just when they emailed what's wrong with my zip-structure, I got blank msg), and later I couldn't get a reply on that matter either from Phil or a new guy that's been announced as addition to their team, so I quit trying..
Realitycash seems promising (lots of content) but their image slice rules make it very time consuming (slice filename has to match set/pic name etc) if they change those to "just put together a list of used content sets/pics in a text file" I'll be making some tours for them too
I cannot find anything on their site but I'm interested, could you point me to their BYO(something) area?