From my few MP matches, I take it that it boils down to three things:
1 - Cheaper
In Multiplayer, you don't get to freely blob. As such, money takes a fair bit more effort to come by. You also tend to have large periods of diplomatic lockdown when you have solidified alliance blocks and aligned interests, in which sustaining a huge army, even at low FL, is rather bad.
2 - Stackwipes
Players are more willing to give up when they are defenseless than the AI, which generally only surrenders when the warscore is low enough - a player having his stacks wiped in a solo war typically means a surrender request is coming. Players use unconditionals a lot more often, too. And because players spread their army to prevent you from just rebuilding a large stack of mercs after a wipe, stackwipes are very lethal in MP. Anything that helps you stackwipe - or prevents you from being on the receiving end - finds itself being massively important in MP.
3 - Coordination
When playing MP, ally reliability is much lower than in SP, however if they do join a war in your side, you basically have yourself an ally that (likely) won't send their stack alone into the enemy capital level 8 fort where their deathstacks are hiding while the rest of your men are still on the border. As such, it's not uncommon in skilled players' games to see a DoW immediately after ONE person took an ahead-of-time mil tech for a morale/tactics advantage. Quality armies sinergize a lot since whoever engaged gets to dictate his side's base morale and miltacs.
There's a fourth factor, but it's pretty minor when compared to the rest, so I'm putting it separately:
4 - Roleplay
It's often downplayed, but it usually quite an important silent factor in both MP and SP, on any Paradox games. Assuming the players aren't going to favor what is "cool" over what is efficient every now and then is just wrong - they do, more often than they should. While Quality is typically "noob-bait" in SP compared to Defensive/Offensive/Quantity for this very reason, in MP it's meta-efficient enough to justify picking it up. Mildly understandle, since everyone can dig the thought of 1 2 5 % D I S C I P L I N E at least a bit.