Because with a strategically important platform like the J-20 you want as many domestic components as possible especially the powerplant. The WS-15 maybe 5 to 10 years away. The last I heard is that only a technology demonstrator has been fired up. They are still working on the prototype. Its going to take along time before we can see the WS-15 in production.
I don't think we know enough about WS-15 to be able to project how far it is away.
That said, if they have a reliable supply of engines (even if Russian) and able to overhaul it themselves, and if a domestic engine currently cannot offer any meaningfully better performance, while also requiring additional time and money to test the airframe for a domestic engine (not to mention divergent logistics), then I think there is a case to argue that there is very little benefit in even using a domestic engine for a "strategically important" platform even like J-20.
After all, the Air Force seems in no rush to re-engine J-10s, likely for similar reasons.
And why didn't CAC use the WS-10 from the beginning ? Or at least from 2011 onwards. And work with Shenyang to modify the engine for the needs of the J-20 as a interim salution untill the WS-15 is available.
Changing to a WS-10B variant now would make sense if there was a projected risk that there would be any sort of projected risk of Al-31F series 3 deliveries being interrupted in the timespan that the Air Force envisages that they would need deliveries of those engines for.
I think we need to remember that even a WS-10B (or other WS-10 variant) would still be an "interim" solution for the J-20 powerplant, and it would be a solution that would require time and money to test and integrate, with different logistics chain to the existing powerplants, not to mention be less mature than Al-31F Series 3 overall (simply because the Al-31 family has been around for longer).
So I think unless there is a risk that the existing engine supply will be compromised, or if unless a domestic solution offers significantly greater capability, (or possibly unless the development of WS-15 takes much longer than originally projected), then it makes little sense to invest time and treasure into what would amount into what is essentially a new, less mature, solution that will only provide a similar interim capability to the existing powerplant.
(as for why didn't they equip later J-20 prototypes with WS-10 -- probably a combination of the same reasons I listed above, along with the likely fact that uprated WS-10s at the time were probably still not as ready)
