Interesting topic. Perhaps the simpler annswer is that having a bigger brain is always positive and as (certain) species get more calories it’s better to “spend” them on brain versus brawn.
All credit to the book “Catching Fire: How Cooking Made Us Human.” We are the only species that cooks our food and gets the 5x calorie boost from that, and the only species with the large brain.
It's not always positive. Because of our large brain we must give birth before the child is fully develope and then it take 15-18 year for it to become adult, before that the child is pretty weak.
One might see this as a positive feed back loop. One thing a large brain is really valuable for is having ideas about the future you want and then acting in the world to achieve that goal. The farther in the future your goal is the better brain one needs. Thus having to raise a child successfully for a long period of time will help evolve even bigger growing brains and longer childhoods.
On the other hand, maybe humans are now breaking this restriction as more and more births are by C-section. I could see a future where most births are that way (if we don't get artificial wombs first).
All credit to the book “Catching Fire: How Cooking Made Us Human.” We are the only species that cooks our food and gets the 5x calorie boost from that, and the only species with the large brain.