With a better product not hamstrung by other interests (ie gouging w/ proprietary storage, over designing and adding something like the back plate that few titles used effectively) perhaps they think they can erode Nintendo's hold on the market. They've watched them fail pretty spectacularly with their last couple of console efforts (Wii was a flash w/ no longevity, Wii U struggled to gain traction w/ anyone other than the faithful) perhaps they think they can leverage the PS4's success to move back into mobile also.
If they'd launched a Vita revision that had dual triggers and clicky sticks I'd probably still own a PS4. Fix the PS4's short-sighted streaming implementation via the new PS4 revisions, release a competent handheld that's focused on the gaming as much as the bottom line and it could be a very successful platform IMO.
With the Vita it was probably too expensive to pivot, if they were going to put forth the effort it was going to go towards a new product.