Probably because of similar reasons as to the Americans' F-22 export ban decision - The J-20 is the best China has, comprehensive capability-wise.
@siegecrossbow have explained very well just above.
Besides FME threats, the other major issue is just a simple lack of bandwidth and outright human capital at CAC for manufacturing J-20s for export.
Meanwhile, unlike the F-22 which entered the game too early - The J-20 is continuously upgraded and improved upon. So while the F-22 gets left behind by their smaller counterparts F-35A/B/C, the J-20 always retain its tip-top comprehensive capabilities in the PLAAF relative to the other fighter models (prior to the induction of the J-36 and J-XDS, that is).
If you constantly update and iterate your production design, you're also going to have to constantly update and debug your manufacturing processes. That's going to stress your human capital, especially if they're already pushing overtime to increase output.
On top of that, if China is to export the J-20 anytime soon, it's going to have to be a variant incorporating certain alternative subsystems in lieu of what's onboard the domestic model. That means diverting resources to updating, reengineering, debugging and certifying an existing baseline J-20 iteration into a customer specific J-20 iteration, plus additional manpower for assembling and then staffing a separate production line.
This isn't going to be something the PLAAF will want to put up with until "J-20 overcapacity" somehow finally kicks in at CAC.
We can’t be so sure about F-22s not being upgraded continuously
It’s a private fighter, they wouldn’t just go around announcing new capabilities
Any significant upgrade to the F-22, especially for a meaningful number of airframes, is going to need Congress' stamp of approval to secure funding.
So
tend to be
and not exactly "private:"
The Air Force plans to retire its older, 32 Block 20 F-22s, but is undertaking a program to upgrade the remaining 154 F-22 Raptors with new cryptography, an expanded open architecture, new weapons, the infrared search and track sensor, and a “Project Keystone” effort to install an advanced threat warning receiver.