It's relatively easy to learn to fly with a thermal glider for one very good reason. They're made to fly by themselves. Thermal flight involves tactics, not really flying prowess. The thermal birds are very stable if left alone, and to fly most efficiently, the pilot leaves them alone as much as possible.

Back during my glider days, it was common to teach a guy to fly in a morning. He'd be solo by the afternoon. Solo everything: launch, flight, landing. That is, if he brought the usual 2m as his first glider.

Gliders also have very few hidden traps. No engine run that has to be tuned to be reliable and that can ruin the flying for example. Nothing much to go wrong or that can be setup badly.