It's probably a hardware limit. Lots of games come up with creative ways of imposing unit caps. Everyone knows gamers want epic battles with massive armies on both sides, but the game slows down to a crawl, especially on certain machines.
I'd like to see an "Epic" checkbox in Game Options, personally. I mean, if you know everybody is playing on a 3 GHz Intel I7 with 8 Gig of DDR3 and an ATI Radeon 5970 on a 12 Mbps internet connection, it would be nice to be able to spam 50 Halcyons with Anime Tempest.