And why can't they fill those 2,227 heavy and tractor-trailer truck drivers positions?  

1.  All of them require passing and continuing to pass drug screening tests.
2.  All of them require expensive driver training and passing the CDL test.
3.  Many of them are 'long haul' jobs  where family is seldom seen.
4.  All of them are high stress,  including  long hours, difficult time schedules and trying to avoid the idiot non-professional drivers they have to share the road with.  
5.  The weather is not your friend (chaining up all those huge tires to go over the pass in the winter comes to mind).
6.  The opportunity for advancement ("getting off the road") is very limited.