It must have seemed a good idea at the time - inexpensive, easy to install. Add a barrier to keep objects from falling from one parking level to another and causing damage in the process.
The only way may be to institute a routine maintenance program to manually remove the trash to prevent it from collecting elsewhere.
An alternative solution could have been an angle attached to the top of the slab. The angle would allow the trash to be swept or washed away without becoming trapped and displayed for all garage patrons.