Sony gave an official explanation of the reasons for blocking the activation of PS Plus cards.

PS Plus Membership Stacking BLOCKED by Sony! - PS Plus & PS Now Voucher Conversion Charts, WHY!
This is not a random failure in the functionality, as many could think, but a fundamental solution aimed at combating the possibility of getting a more expensive PS Plus Service, which will be introduced in the near future, due to a cheaper relevant subscription.
Information about this appeared on the official PlayStation page.
Obviously, Sony strives to prevent the players in every possible way to accumulate a prepaid subscription time before launching an updated service.