I think the popularity of Rodinal and HC110 is due to the fact that they are usually diluted from the bottle and used as one shots, and that makes them ultra-convenient. They also last a long, long time in the bottle. They also both work very, very well, and are both very, very versatile. And the higher dilutions that these two developers will work at give longer development times, and this gives a real boost to the confidence of sloppy, lazy twits like me who object to being fanatical about timing things.
It takes a good imagination to see the difference between an HC110 negative and a D76 negative, but mixing up powder is not something I'm fond of, otherwise I'd be using XTol more.
Rodinal is said to have it's own definable quality, but I use it only when I want lots of grain in a half-frame format. Not that it's particularly grainy, but if you agitate constantly with this developer, it does produce more pronouced grain than HC110 or anything else I'm aware of. I have used it at 1:50 with HP5 and FP4, and handled exactly like I use HC110 (at 1:63) for 11 minutes, they look substantially the same - both produce beautiful negatives that print well.