It's intended as an inefficiency cost. Actually taking Warlock levels gives you way more invocations, but you pay for it with, y'know, not getting levels in something else. Taking Eldritch Adept is much more efficient, but consequently more limited. Likewise, spending one of your Invocations on (to use the 5.5e term) "Lessons of the First Ones" is a way to pick up a feat...but it can only be a feat of relatively restrained scope, not just any old feat.
Getting two invocations for the price of one feat would be far too little. Getting one invocation is, yes, probably too weak--but it's hard to see what else it might give. A short-rest spell slot would be too powerful, and Warlocks have stronger cantrips, so eldritch blast kinda nixes that as an option. Other than invocations, there isn't much left to the class.
I will, however, argue that Invocations are not the only reason, though they are the biggest. The other reason is, as you might expect, short-rest resources. You have to economize, but not so much that you're pinching pennies. That's an engaging mechanical space.