Spinners. Any moron with a bat can tonk a bad spinner. A bad fast bowler can get through on his pace against weak batsmen. That's why fast bowlers rise through the ranks so quickly - you bowl quick and maintain a half decent degree of accuracy, and you'll get through most batsmen's defences. And when the pitch aids you with bounce or swing, it's even easier. On flat pitches, your pace will be more than enough against age-level group batsmen who normally suck.
For a spinner, unless the pitch is turning, you have nothing. You are a slow bowler, and every batsman can face you with no problem. This leads to some spinners giving up on flight and bowling flat, cause even tailenders can tonk a bad flighted delivery.
Spin is harder in general. It takes a lot of guts and brains to be a good spinner, whereas a good quick bowler isn't hard. It is considerably harder to become an international level bowler as a fast guy, as there is more competition, and once the batsmen get better you need to develop other tools to go with pace. But once you've cracked first-class level as a frontline spinner, all you need is experience till you are ready for NT level.
That's why you'll see a lot of Nat spinners have Test averages closer to their FC ones, whereas Nat fast bowlers tend to have Test averages fair bit higher than FC ones.
