1. Volition was to lazy to put it in.
2. They felt the weapons where to powerful for a competitive multiplayer use (i.e. Rocket Launcher, Magnet Gun, Singularity Cannon, Plasma Beam, and Plasma Cannon).
3. Since the single player is set underground they felt like they couldn't make any underground multiplayer maps.
4. They wanted to play it safe and not include it.
5. They found out that they forgot to put it in and made up an excuse.
6. They wanted to take a gamble and see where it would take them.
7. The team that made the multiplayer for RFG felt they couldn't be as or make a better multiplayer then RFG.
8. The Hardies would take over RFA and as they did RFG.
9. Single player didn't wouldn't connect to the multiplayer aspect.
10. Wanted a game mode that would show there new destruction engine better.
11. They knew they couldn't compete with the big name titles coming out (i.e. Gears of War 3, Modern Warfare 3, Uncharted 3, Resistance 3, Battlefield 3, Twisted Metal, Brink, and etc).
12. Forgot to put in RFA servers for multiplayer.
13. Felt if they made a bad multiplayer experience that people would not recommend the game and that it would cause sells to go down.
14. They spent more time making the single player better that they felt they didn't have time to make the multiplayer for RFA.
15. They thought if they put multiplayer in RFA everyone would forget about RFG.
I know some of these reasons are stupid but out of these 15 reason which 3 is the most likely reasons.
Mine are 1,11, and 14