The simplest answer may also be the best: there's a lot to learn. And it seems that as society becomes more complicated, there's more you must learn to survive.
There is a strong evolutionary advantage to taking care of our sick and elderly -- experts are very hard to train. As medical care improves we can afford to invest more time (childhood) and energy into training increasingly specialized experts.
It seems like a positive feedback loop. Improved brains require slow development and allow for greater social complexity, which requires bigger brains and even slower development.
To summarize, this article adds that perhaps slow development contributes to the loop, rather than detracts from it.
There is a strong evolutionary advantage to taking care of our sick and elderly -- experts are very hard to train. As medical care improves we can afford to invest more time (childhood) and energy into training increasingly specialized experts.