Simple. Money.
The J31 is a private venture by SAC, after it lost the PLAAF next gen competition to CAC.
All the Chinese government resources went to CAC to finance the development of the J20 in the wake of that decision.
SAC are effectively on their own in this, and 5th gens are notoriously expensive to develop and build.
I think they are stuck in a catch 22 trap.
SAC doesn't have the financial power to fully fund the development of the J31 on it's own, no single company does. It needs investors to jump in with money to keep the programme running, but no one looks remotely ready to jump in and help fund the programme since they know its stuck.
I think SAC was banking on getting the same reaction as Sukhoi with the PAKFA. Get a prototype in the air, generate enough international interest that a client signs on straight away with enough financial commitment to get the programme moving, after which point it will generate enough momentum and further commitments to become self sufficient and viable.
Problem is that SAC isn't Sukhoi. Not only does it lack the history and brand of Sukhoi, it also doesn't have an internationally marketable product.
Sukhoi funded the PAKFA on the back of hundreds of Flankers sold world wide, as well as directly contributions from the Russian government, and then India.
I think SAC is bound by Chinese government agreements with Russia to not sell any of the Flanker variants it produces.
While it did sell hundreds of Flankers to the PLA, it cannot expect anything like the profit margins from those sales as Sukhoi could from international exports.
SAC also doesn't have much, if any, financial contribution from the Chinese government or international customer.
They pretty much spent all their retained profits from its decades of Flanker sales to the PLA on the first prototype (many they have enough for a second, but almost certainly not enough for more than that), and now just doesn't have the income and operating profit margins to fund it at anything like a normal pace.
They are probably piggy backing its development on other projects, since it cannot afford to fund a dedicated team. That's going to massively slow things down, since the teams will need to deliver on the programmes that are actually paying the bills as their first priority. So the J31, at this stage, is very much like an after hours hobby for SAC.
