There are different definitions of what to call "stealth". Some aircraft like the B-2 are total all-aspect stealth, others like the F-15SE proposal only make small RCS reductions.
The Su-57 is in between. It's a stealth design, but it focuses on forward aspect stealth. This is similar to the F-35, which also forgoes rear hemisphere stealth. The Su-57 applies major RCS reductions, like eliminating vertical surfaces and shielding the engines, but still builds upon the Su-27 Flanker platform, so they couldn't completely rearrange the planform.
The reasons are historical. The Soviet Union's program to develop a 5th-gen fighter started in 1979, 2 years before the US ATF program (the F-22). Both programs wanted a low-observable, supercruising, supermaneuverable, fused-sensors fighter. Here is what the US considered:
The USSR also had all its design bureaus compete, but I can't find open sources for their designs. In 1987, Mikoyan won the competition with their Project 1.44. Sukhoi, Yakovlev, Ilushin, Antonov, Tupolev lost. In 1991, the USSR collapsed, and the program was defunded. Russia was considering joining NATO, or disarming completely.
Meanwhile, Sukhoi's Su-27 was raking in the cash. It briefly overtook even the F-16 as the most-exported fighter, though not for long. Heavy fighters usually suffer from poor sales, but the Flanker's fly-by-wire, supermaneuverability, payload, reliability made it popular for export.
Sukhoi knew the future is stealth, and invested into developing their own design further. In 2000, when the new government revived the 5th gen program, Sukhoi had a prototype ready to go. The program took just 10 years and $1 billion from that to first flight in 2010.
The only way Sukhoi could accomplish this was by reusing the Su-27 as much as possible. Early Su-57 prototypes were just modified Su-27s, testing one thing at a time. Later, they changed the airframe, but they had to keep the overall layout.
Some prototypes tested flat combiner nozzles, suited for deep stealth. Some had complete S-ducts and blended nacelles:
None were picked for the production version. S-ducts are heavy. IR-lined flat nozzles are even heavier. This results in a notable drop in performance. The F-22's useful load is only 0.48 of MTOW, down from 0.58 for the F-15. That's a massive drop, and part of why the F-35 lost the stealth nozzle. Sukhoi didn't put as much trust in stealth as Lockheed did, and wanted a plane that would work well even if stealth failed.
The practical fact is that the Su-57 is a transitional design, rather than a new all-stealth concept. That's not necessarily a bad thing - stealth is as of yet untested in peer-level combat, and it can be countered. To that end, the Su-57 has the most comprehensive counter-stealth sensor suite to date, with two IR systems and a dual-band radar, which might offset its higher RCS.
From the top, the Su-57's design (left) is consistent with stealth, if following the concept of low-level penetration. But Sukhoi's all-stealth design (right) is different:
The Su-57 makes compromises. It can take different engine models, be quickly modified for A2A or A2G, uses intake radar blockers rather than deep S-ducts. Stealth-wise, while the B-2 and F-22 are estimated at roughly -25 dBsm median RCS (-40 for head-on aspect), the F-35 at -20, the Su-57 only claims around -10 dBsm median. That's a lot better than +10 to +15 of most 4th gen fighters, but it's not state-of-the-art stealth.
The Su-75 (on the right), slated to fly in 2024, doesn't compromise. It's designed from scratch, has no predecessor, follows every rule of stealth design there is. It was started in 2000s Russia, enriched by high oil, global trade, and Sukhoi's own export success. Yet, it doesn't yet have a single customer signed up.
The reasons are trust, reliability, familiarity. It will take a long time to get acquainted with the Su-75 and iron out all the kinks. Even longer to learn how to keep it stealthy - a challenge in itself, which took impractical restrictions for the B-2 and time for the F-22. India and China realize it will take a long time to learn, and China's more eager to invest that time in their homegrown platform.
On the domestic side, there's the cat-and-mouse game between stealth and counter-stealth. Playing is definitely better than not. But Russia has previously claimed the S-300 to have strong counter-stealth. If true, it means the Su-57 and even Su-75 won't be able to rely on stealth in former Soviet countries, which are stock-full of S-300's, originally meant to guard against US/NATO. This might be why the compromise option was picked for the PAK-FA over full-stealth prototypes.
Also, it could be that the shape works better than it seems to. But Sukhoi's own patent only claims -10 dBsm. Here is an independent evaluation of the Su-57's RCS, compared to F-35 and J-20. The conclusion of this modeling is that the Su-57 has about 10 dB worse RCS than the F-35, and its engine placement plays a role in that. It is, however, a major improvement over 4th gen fighters.
Summary: Sukhoi hedged their bets with the Su-57, building a plane that can sell as either a stealth fighter or as a better Flanker. It doesn't look as stealth-oriented as the F-35, J-20, or Su-75, because it isn't. But from key aspects, its RCS reduction is significant, about 100-fold.
