We could simplify it even further: it's all about intellectual stamina.

From my personal experience and from my observations of others, the limiting factor to learning is the availability of mental energy to persevere/endure in the learning process.

You have a problem. Either an academic homework problem or a real-world problem of some sort (think error message in the console). You try one thing but it doesn't work... then another, and still you're not making progress. You're starting to realize solving this problem is not obvious and will require some "brain sweat" and you have to make a choice whether to invest the hour (or day!) of your time to learn enough to solve your problem.

Learners willingness to persevere in their efforts to solve the problem depends on lots of factors, but most importantly they are conditional on the level of learners' interest/motivation (intrinsic or extrinsic). Ideally, learners will be motivated purely by the "knowledge buzz" of learning new concepts and seeing the connections between them, but any source of motivation is OK: as long as it pushes/pulls you enough to go through the necessary learning and solve the problem.

I recognize that learning to learn is a useful multiplier, but learners need to have base rate of (1) intellectual stamina (brain muscles?) and (2) interest/motivation to push through. Otherwise learning is not going to be happening.