Flat plains maps are
interesting when they pop up once in a while, because you see forced coordination with shield walls and "formations" (also known as "Big clump of people behind shield wall").
The problem is that it just isn't
fun for any length of time. Given balanced teams, the optimal strategy is to have your infantry remain stationary in a big pike studded shield wall formation, with archers in the middle. Moving that clump is discouraged, since it breaks up the formation, making you more vulnerable to enemy archers and cavalry.
The end result is nearly always like Gurnisson explains: Infantry sits on their ass for 4 minutes, while archers and cavalry work to achieve cavalry superiority against the opposing team, at which point the armies clash due to time limit, low ammo or boredom. In the ensuing chaos, the team with cavalry superiority is almost guaranteed to win, as they can weave back and forth through the broken lines picking off enemy infantry.
As cavalry, this series of events is exciting. They get to start out doing cavalry duels and trying to identify weak spots in enemy formations. Towards the end, they get to run rampant in broken lines of enemy infantry. As infantry, it's about as much fun as watching paint dry. Once the initial excitement of "Woo, formations!" wears off, you're just sitting on your ass for 4 minutes watching other people play the game, before partaking in a short chaotic melee that ultimately isn't even that crucial.
Even ignoring the obvious balance problems if one team gets more/better cavalry (in which case they're almost guaranteed to win), flat plains maps just aren't fun in the long run.
Disclaimer: I've played such maps as lancer, as shielder and as archer. Only the lancer had fun after a couple of rounds. (But indeed: The lancer had a
ton of fun.)
The variety we should strive to have in our maps should be in-map variety, like Sieweed explains. Maps that are mirrored and allow all types of players to succeed. In my eyes the best example of this is
THIS MAP. It is mirrored, and has well suited fighting positions for all styles of play. It has favorable positions that are worth fighting to reach, but aren't unbreakable once attained. The deciding factor is the sum of all the team's strengths, not only cavalry (or archers, or melee infantry).