E3 has become very costly for companies, even for the likes of someone like Sony. I'm guessing they are not seeing any ROI for the money, time and overall investment they are putting into the whole E3 show.
Felt weird being there last year and witnessing the room PlayStation used to occupy being used as a storage space facility.
My biggest problem with Sony skipping E3 isn't with putting their first parties at another show, but rather not giving a place for third parties to show their stuff. The vast majority of games sold on their system come from third parties and it seems like they're hanging third parties out to dry by not giving them a place to show, when everybody else will be showing their games.