i dont think there is one reason specifically, could be a majority of reasons. here are some of my thoughts
after last year with Sony having both marketing rights to CoD and Battlefront, MS thought it should step up its marketing efforts. they probably out bid Sony, by a considerable amount too, considering the success of battlefront
EA maybe thought they didnt want to share the spot light with CoD, so they opted to go for MS which would market it as THE fps of the year
Maybe Sony secured rights to market Titanfall 2, with TF2, CoD and Destiny (a big expansion will come out this year) they thought no need to spend more money on battlefield because they are covered in the FPS genre
maybe Sony's contract with Activision to market CoD prohibits them from promoting its main competitor which is battlefield (i find this least likely though, but still a possibility)
E3 is yet to come, and we dont know what other big 3rd party games are coming out, Watch Dogs is all but confirmed, and Sony might have the marketing rights for that and most likely do. so they might push CoD and Watch Dogs, maybe other games as well, and felt no need to spend more money on marketing big games
