Because conductive heating principle: Trays direct load on shelves, close contact with them. So, material can get a higher temperature uniformity. And the trays loading are manually. Smaller FD with less trays quantity is ok, but for large FD will hundreds of trays, is not convenient.
Large FD use radiative heating to reduce labor and save time. The trays quick loading/unloading assist by trolleys. After placed in drying chamber, trays will be suspending between 2 heating plates.
The typical applications of conductive heating technology, which used in freeze dryers machines manufacturing. FYI, pharmaceutical freeze dryers, commercial small food freeze dryers.
Typical applications of radiative heating technology in freeze drying machine build, is the industrial large food freeze dryers.