I'm not getting why people think they could of branched out. If it was an American company sure I could see it but we're talking about a small Japanese company here. To reach the biggest market they could on a small budget they put games on the most popular system which was Sony's. Nintendo's systems were under powered compared to Sony's, the DS couldn't even play a PS2 era Disgaea 1 without stripping out features. The Xbox was never an option due to the tiny install base they have there especially with the smaller budgets that they're working with.
The only non-Sony game they've made themselves was Disgaea for the DS the other 3 were just published by them.
As for putting 5 on PS4 only, it's basically following a trend of putting games on the now current console a year or two after the console debuts and with one near the end of the console's lifespan. Not to mention with the features that they're putting into the game the PS3 can not handle it hence why there is no PS3 port.
Playstation 2 released in early 2000, Disgaea 1 released in 1/2003, Disgaea 2 released in 2/2006
Playstation 3 released in late 2006 Disgaea 3 released 1/2008, Disgaea 4 released 2/2011, D2 released 3/2013
Playstation 4 released in 2014(Japan) Disgaea 5 release date 3/2015.
If they are leaving then they are going out with a bang since 5 is their PS4 debut and can only be played on the PS4.