There will be more free roguelikes in the future, but basically the problem is that the major ones are too big to really do them justice.
For example, I might be able to say something about crawl but I haven't played any of the other big ones long enough to have a really educated opinion, and I think pretty much everyone on the team has played maybe 1 or 2 of the major ones for long enough to really know them well (and it's different games for each person).
With the binding of isaac (or 100 rogues, etc) I feel perfectly comfortable talking with some authority after playing the game for several days.
Moreover, the facts are that the commercial releases normally put much greater emphasis on easy to learn controls and interfaces and are a lot easier to get into (and therefor to play in time for the podcast).
That's probably a side effect of what you mentioned at the end of your comment, but honestly when you have to play a new game every week for many weeks straight you don't want a game that take a week just to get used to its quirks.
The best solution I can think of is to have anyone who is interested in joining us to post it here and we could integrate "experts" into specific episodes (e.g. you might be up in an episode about IVAN, someone else would join in for the nethack episode, etc).