Battle requires less teamwork than siege.
If you kill someone in battle, he stays dead. You just helped your team, no matter where, how and why.
In siege, killing someone has no value in itself, unless it happens at the right time in the right place.
This is also the reason why most pubbers, including me, don't play siege often - the amount of teamwork needed for the game to be REALLY funny and challenging is more or less impossible to achieve. That's why battle is better suited for pubbers.
I still hope I can pull off the most epic gamemodes of em all:
2 opposing forts, more or less a lot of land in between, capturable spawn points in between (so inf doesn't have to walk that long), buildable siege equipment, gamemode: capture the enemy base while you have to defend your base.
I will call the gamemode "Epic".
Edit: if you want to help me create a prototype map for Epic, pm me.