[Custom Battles] Why are the rosters so narrow?

[QuintusSertorius]

I know there's a reason to limit what's available in custom battle, I just can't remember what it is.

By which I mean you load up a custom battle, and the troops available are very few. You don't have all the recruitable units for your faction, and none of the mercs you'd be able to get in the game. Which makes it pretty difficult to try formations and units out when they're not there.

So what gives?

And the follow-on question, what do I edit in the EDU to make them available?