Because the F-35 design is most optimized for SVTOL, and pays the price in terms of acceleration, range, and max speed.
The Chinese engineers may not have had a choice because the F-35 is a highly draggy design that needed 40,000 lbs of thrust.
But two engines take up more space, leaving less room for fuel. The F-35 with 8.5 tons of internal fuel gets only 590 nm combat radius; and it is actually failing to achieve that range target. If the Chinese F-35 has two engines, then its fuel capacity is even lower.
The better solution was learn from the F-35 data and design a new airframe that fixes the design compromises of F-35, but the Chinese engineers decided to not to take that path, as it appears that stealth and time to market were more important than the most optimal airframe design.