Well, from my understanding the "MAIN" reason would actually be the way each engine handles DML sessions. I honestly can't say InnoDB lacks in the performance department in regards to large amounts of data. However, it does allow for multiple/parallel DML sessions, of which would be a negative effect on rA.
MyISAM will currently limit those DML sessions to run 1 by 1, meaning that the rA system, only has to process one of those at a time to bring forth an update, thus reducing the strain on the server and lag in total. Then when you compare how scripters use SQL to store information, you'll see most of it is called when player performs action X. And since we can have anywhere from 1 - 1000+ players at any given time, you have to take into consideration that each of those players could trigger action X at once. Or in the instance where action X is killing any monster and updating the data to log how many kills... Well, what happens when a 10 wizards in 10 different parties kill 100 monsters each with SG?
The answer is you'll have 1,000 indivisual queries to run, now if all 1,000 of those run at once, then rA systems will suffer until they are done and you'll experience lag on your server.
But if we were to use MyISAM it'd run all 1,000 1 by 1, meaning the lag experienced should only be that of Internet connection / cpu stats.
Again, this is all from my understanding. So I should say that, certain DBs should allow for InnoDB while most that perform at a more frequent rate, like that in my example or saving all the characters, should be handled by MyISAM.