Because that was not a winning game for them, and there was no need.

The US snatched up German and Japanese scientists at the end of WWII with "Operation Paperclip" et al, but the USSR did the same thing. If anything the Soviets were even more aggressive about it, with "Operation Osoaviakhim" involving taking thousands of German scientists and technicians from occupied territory for forced employment in the Soviet Union.

Indeed, a huge number of early post-WWII Soviet technological achievements could trace direct throughlines to this effort. For example, the R-7 family of rockets (Sputnik/Vostok/Soyuz launchers) are even today extremely close relatives to the V-2. The V-2 (A4) rocket uses liquid oxygen and highly concentrated ethanol as propellant along with a turbopump powered by high test peroxide. The Soyuz first stage rockets use liquid oxygen and kerosene as propellant with a peroxide powered turbopump, substantially the same design as the V-2 just scaled up and multiplied.

Also, captured German and Austrian scientists developed the Uranium enrichment centrifuge that became a cornerstone of Soviet nuclear technology in the 1950s. This, in particular, is a good example because one of the lead engineers on that project (Gernot Zippe) ended up leaving the Soviet Union and bringing the technology (which has become known as the Zippe centrifuge) to the West after the scientists had been released.

By the time the space race was heating up the fact of the Soviets having forced captured engineers to work at effectively gunpoint was becoming known, and there was little advantage to the Soviets of highlighting such events. Pointing to the example of von Braun would just point back to the Soviets having done the same or worse.

More so, in the early years of the space race the Soviets were broadly seen as being ahead, they had the first orbital launch, the first satellite, the first crewed flight, the first spacewalk, the first lunar probe, etc. It was more in their interest to keep the public perception of the space race at a level of accomplishments at that point. And by the time the Apollo Program started to turn the tide it was too late, at that point a personal attack against von Braun would have come off as a sign of weakness.