I'm fairly sure that it's all to do with the money MO generated post-launch.
Although it was a dick move by bioware to tie the MP of ME3 directly into the SP it did mean that more of the playerbase would play it, in order to get 'the best ending' (which we no know is redundant) & add to that, that all weapons were upgrades & not repeats would mean that, as soon as you have all guns at lv1 you were guaranteed to always get a stronger weapon, this encouraged players to perform micro-transactions. Side note, I know a guy that, when every ME3MP dlc dropped he would spend £50 on micro-transactions in oder to get 'the best guns' but he never did (I actually got the Lancer & Venom before him) .
Now, although there are people playing this game who have bought platinum, I would hazard a guess that it's not nearly as much as ME3MP, particularly because of the horrendous mess BW made of the loot system, this would mean less money going into BW from the multiplayer, so less cash for content, thus, less content.
Just my theory.