Can anyone tell me why did it take this long to develop this fighter, when the J20 was started at around the same time, and it is now in service where as this is nowhere near complete?
Because the F-31 is a private venture by SAC and has not receive any government funding. At this point of time it seems that the PLAAF has little interest in a second stealth fighter and is focused on bringing the J-20 to full capacity first before embarking on any other projects. Regardless of whatever rumors that are floating around that the PLAN is looking at the F-31 as a carrier fighter.
And from SAC pov it is pointless to aggressively pursue a project that has insofar received little to no government interest, and if the government actually does in the future it will very likely order CAC to assist SAC in the project with their experience with the J-20 which will save SAC a boatload of cash. As of now the company can still rely on the Sino-Flankers production line to generate profit.
So SAC is most likely keeping the project on a life support, funneling in just enough cash to keep the project alive in hopes that Beijing with have an interest in it but not enough as to become a major expense.