Profitability.

That is the only true answer you will get in this thread.

It limits how much progression can be derived from a purchased booster, which in turn gives more incentive to purchase additional boosters.

The other option was to remove active SP entirely and make it completely passive like EVE.

Either way, for them to make any kind of profit it had to be done this way. If I have a certain favorite fit, I am unlikely to purchase any kind of aurum equipment once I have skilled into it. I will, however, be likely to purchase boosters as that can give me access to other options and further optimize ones I already have.

You should never assume a game designer does anything because it is what a player suggests.

There were just as many people who didn't, and those with "a life" decided that they did not want to be victims to people who could grind 24/7. And some wanted it to completely mimic the EVE system, but that wouldn't be as profitable because this way they can sell twice as many boosters.

In the end, it came down to "What is going to get us the most money while pissing people off the least?"

This was the natural answer. But the money part always comes first. The desires of the community are going to be an afterthought. CCP has often struggled with that problem, and they have some good voices on the inside who raise that red flag when it happens thankfully. But it does happen.