Why more experience is required to fly the airliner above I assume 6096 meters? I would expect, taking off and flying close to terrain should need more experience.
I'm unaware of any regulation that requires a minimum number of hours to be a captain or first officer above FL200. That doesn't mean there aren't such, just that I don't know of them.
It's not that that taking off and flying close to terrain requires more or less experience, but that safely operating at high altitude typically involves a different set of knowledge and experience than operating at low altitude. A seasoned bush pilot who does only that would be ill equipped to suddenly become the captain of a 747 at 35,000 feet, just as the 747 captain who had never been a bush pilot would be ill equipped to scud run through a valley to land on a remote airstrip.
Part of the difference between two such pilots is, of course, a matter of a propeller airplane versus a jet, a slow airplane versus a fast, and the differing complexities of their aircraft. In addition to those differences, though, the environment that they typically operate in and indeed the altitude itself makes things different. The nature of the emergencies they can possibly face are also different.
Offhand I can think of three things the typical low altitude pilot doesn't have to contend with that the high altitude pilot does. There are certainly more.
Oxygen considerations, in other words what you need to do if you suddenly don't have it at a breathable pressure. Probably never going to happen, but the high altitude pilot trains for it.
Regular operation in or close to the coffin corner. If you're unfamiliar with the term, Wikipedia has a reasonable explanation as well as
A whole different situational awareness environment than the low altitude pilot. For example, I used to do a flight from Sao Paulo to Miami. Two other carriers had flights scheduled that went out immediately before us, also 747s. Everybody was always heavy, which meant the first guy out got the highest cruise altitude available for the weight we were all at approximately, the second guy 2,000 feet below that, and then us sucking fuel 4,000 feet lower than the first. However, I knew that if I could open up the distance between myself and the second guy, I could get a clearance through the second guy's altitude to the first guy's altitude since there was enough time/distance between us. So what I would do was to climb at best angle rather than best rate. Then, if the weather was good (i.e. no turbulence) and I had the engines for it, it took very little fuel burn before I would able to get 2,000 feet higher if I was willing to accept slightly lesser protection from the top of the coffin corner. Sometimes that would work out, sometimes not, but it always got us at least up to the first guy's altitude. And on rare occasions, it meant we reached Miami before either of the other two.