I'm going to attempt to answer your question, but before I do allow me to admit I don't know why they did it, but I will offer what I think (opinion only).
When Nashville, Atlanta, Minnesota and Columbus entered the league, each team paid $80 million. That meant that the owners got ($320 million / 26) each. For $12.3 million each, they each lost 4 players.
The Golden Knights knowing how it worked last time, and how long it took those teams to become competitive, negotiated a better deal. The asking price for a franchise was $500 million. They negotiated the entry draft rules with the NHL in exchange for their $500 entry fee. Now, in exchange for giving up one player, the owners get $16.7 million.
VGK were just unwilling to part with $500,000,000 plus all other startup costs unless they had a better shot of being competitive sooner.
As for stipulating between service times in certain leagues, that really wouldn't work as teams would be stuffing certain Josh Anderson types in the AHL to not expose him (or vice versa). That had to do it with an eye toward the Collective Bargaining Agreement. After all, the NHLPA is getting 23 (or more) new dues paying members next year. Forepar is correct, that's just the way it is. But hopefully it helps you understand the why.