There are a number of possible reasons (we wouldn't know unless the PLAAF tells us and they probably want to keep their actual motivations secret from the Russians anyway)
First, the original contract was not "to reverse engineer". China may have wanted to do that, but the Russians were not keen, having been burned on the SU-27. That's part of the wiki entry linked to (At one stage, Rosoboronexport demanded that China issue a legally binding guarantee against copying.) and Sergei's answer also mentions it (+1).
Does China's J-11 Fighter Jet Have Russian SU-27 "DNA"?
Key Point: China wants to build its own fighters and that means using any means to do it. In fact, Beijing has been buying, stealing, and copying foreign weapon systems for decades now.
The Shenyang J-11 is a Chinese copy of the excellent Russian Sukhoi Su-27 “Flanker” multirole fighter. In fact, it was at first an authorized copy—but Chinese ambitions to adapt it with locally produced technology transformed it into a reverse-engineered headache for Russian industry. In successive variants, the J-11 and the Flanker-derived J-15 and J-16 have been at the forefront of Chinese efforts to produce long-range fourth-generation fighters that can contest the seas around China—if only Chinese engineers can work out the kinks in their domestically produced jet engines.
And this was part of the game here:
There are fears China’s decision to procure only 24 fighters indicates an intention to reverse engineer and copy the fighter, as it did with the Su-27SK. In 1995, China secured a $2.5 billion production license deal from Russia to build 200 Su-27SKs, dubbed the J-11A. In 2006, Russia killed the contract after 95 aircraft when it discovered China had reverse engineered the aircraft and was covertly manufacturing an indigenous variant, the J-11B, with Chinese-built avionics and weapons.
Cliff, author of the new book, "China's Military Power: Assessing Current and Future Capabilities," said he was more skeptical of China reaping as much as many fear from the Su-35.
"Just buying examples of technologies, however, doesn't immediately convey the ability to make them oneself," he said. The best example of that is the AL-31 engine that goes into the Su-27 and Su-30.
"China has had access to that technology for over 20 years and apparently is still struggling to make its own high-performance turbofan engines."
Chinese cleverness in research and mass manufacturing is as well-known as their tendency to "appropriate" intellectual property. However, knowing what something is doesn't mean you know how to make it. Modern jet engines by the like of Rolls Royce have all sorts of wonderful arcane metallurgical stuff going on and you can look at them, but you can't look at the foundry that made them. That's what makes manufacturers like Pratt & Whitney, Rolls Royce or Snecma, as well as Sukhoi/Saturn special: decades of expertise and tinkering. Ditto with high-end chip manufacture: you can look at the finished products, but only the likes of TMSC and Intel know how to mass-produce really small chips on 5-7nm technology. And even Intel... hasn't been doing 10nm too well lately.
So that leaves us with another possible reason to buy another Su-35 lot: they want to use them, NOW.
China both will benefit from having Su-35s in its fleet directly. But perhaps more importantly, they can start experimenting with tactical military doctrine built around 4.5 generation fighters. It might be less the actual immediate firepower, or the extra access for copying reasons, than it is to train their air force.
Modern militaries are not just about owning a stable of high-end weapons. By that token, Saudi Arabia would be a military powerhouse. It is about, for decades, taking lots of very smart soldiers, running military exercises and seeing how best to use certain classes of weapons. You don't need the final weapons, but you need something in their class. For example, the Germans were testing tank doctrine in the USSR till the early 30s (great idea, comrade Stalin).
China has also done this, since 2012, with their first, Soviet-built, carrier.
Liaoning (16; Chinese: 辽宁舰; pinyin: Liáoníng Jiàn) is a Chinese Type 001 aircraft carrier. The first aircraft carrier commissioned into the People's Liberation Army Navy Surface Force, she was originally classified as a training ship, intended to allow the Navy to experiment, train and gain familiarity with aircraft carrier operations. Following upgrades and additional training in late 2018, Chinese state media announced that the ship would shift to a combat role in 2019.
The F-35 apparently routinely is out-dogged by F-16s, yet its pilots are still enthusiastic about it ("the other guys are fighting blind"). That requires a new doctrine, avoiding visual combat range engagements to maximize stealth advantages. So a SU-35 may want to close into specifically F35s instead. But again, you need multiple operational planes to practice, practice, practice with, way ahead of time. Rather than a few planes sitting around all disassembled. Citing Defense News again:
"In the mid-term, the development of this family is much more important for Chinese air power than their stealth aircraft programs," the J-31 and J-20 stealth fighters, Kashin said. "Exercising with one regiment of Su-35s will help them understand what direction can be chosen for the future of their heavy fighters fleet, what they can do themselves, when they have to go to the Russians for help, etc."