So what's the logic? How would the Soviet Union gain if the Pope was dead, they would just elect another Pope???
The USSR had survived for six decades before JPII, and though all of the other popes had condemned communist atheism, no pope had ever stood up to them so boldly. Solidarity was a growing threat in Poland and JPII was loved and seen as the greatest inspiration for the Poles.
In short, the communists were desperate. Any pope would have been better in their minds than JPII. Had the assassination succeeded, they probably wouldn't have let any cardinals from the Soviet Bloc out to attend the conclave.
That Pope was Polish and a great inspiration to millions of Poles. He was also supportive of Polish aspirations for spiritual and political independence from the monsters in the Kremlin. The leaders of the USSR have every reason to fear what came to pass in the late '80s or much worse.