While I really enjoy variety in readers, I also appreciate Steve's voice; so, I don't mind him reading many of the stories on EP.
Given that Steve seems to like having other readers, too, I am going to go out on a limb I know nothing about. I'd wager finding good voice talent that also has time and good recording equipment isn't as easy as it sounds. I am not sure if his voice talent gets paid, but if not, then you also have to find someone with literally "free" time to voice a 35-50 minute story, which is several hours of work. For example, I have a stellar voice, am professionally trained, and get asked I am sometimes asked to put in speaking appearances. But, I have crap for recording equipment and my time come and goes. There are 'casters out there who obviously have the voice talent and the recording equipment, but definitely not the time. There are people with the time and top notch recording stuff, but voices so grating cheese runs if they enter the same room. To top all this off, they have to want to read for EP, which they may or may not like (not everyone is into SF, though I have never really fathomed how that happens).
I think Steve likely tries to get appropriate readers, and he seems to genuinely enjoy having other readers on EP. It just may not be as feasible as often as he (and others) may like. JMHO, take it for the less-than-2-cents it's worth.