Frankly, the level of actual academic ability is pretty similar. It may sound unlikely, but both require about GCSE level in maths and physics and they are not very different in that respect.
The big difference is just the sheer amount that you are expected to learn at ATPL level. Most people, even if they are not very good, manage to hack the PPL merely by learning all the answers off by heart. You go out and buy the 'PPL Confuser' and just get down to it.
You can't do that with the ATPL. Not because it's more difficult - it isn't, much. It's simply that there's too much to learn off by heart and, actually, it turns out to be really easier to understand and work it out for yourself than learn it parrot-fashion. That requires that you actually have to understand it.
It's a hard lesson and some people never learn it. They then post on pprune moaning that "the feedback let them down".