Why not have an automatic loading system for vehicle mounted mortars?

they do exist take as example the AMOS system a breech loading 120 mm mortar mounted in a turret with a firing speed up to 26 round a minute thanks to a automatic loading system

The drawback is that it has a relative large price if you planning to buy one

The same reason that US tank cannons are still manually loaded - auto-loaders are complicated and break easily. And a broken loader = useless weapon.

Yup, I watch the military channel.

Just wanted to add another problem with auto-loaders specifically for mortars: Since they are indirect, high trajectory weapons, the barrel is always pointed up. This would require an auto-loading device that either sits on top of the barrel (gravity feeds the round down onto the firing pin) and then clears itself out of the way in time, or an auto-loading device underneath the barrel - in which case it would no longer be a simple, functional mortar. It would just be a complicated, high-trajectory, auto-cannon that fires mortar rounds.

This has been tried in various armies around the world, the problem is the loading mechanism tends to jam at the most awkward moment after a few round.

Of course there are. They are not that complicated. Unable to divulge more info other than its similar in principle to an automatic rifle or machine gun.