From what I've seen through the years, it seems like sweeping is the skill that is much easier to develop than guard passing. A lot of blue/purple belts are really good at sweeps, but not so comfortable passing high level guards.
This may be due to the fact that in a tournament, you can win on sweeps alone (and defense afterwards). If you do not pass the guard but are still up on points, there is no pressure to pass.
Keeping this mentality will only slow down progression though, so especially in training... guard passing should be a focus after sweeps have been mastered.
Now a very technical pass from Marcelo Garcia