Anything above has the risk of dripping onto whatever is below (even if you are careful). Chicken has higher incidence of bacteria growth, spoils faster.

In addition, beef is typically cooked to a much lower temperature than chicken, between 120 and 140 F, which is not hot enough to kill something like salmonella, if the beef has been contaminated from spoiled poultry dripping from above.

The outer packaging is often contaminated, and can contaminate the beef packaging. Bacteria survive in the freezer.

Beef, except for pre-ground beef, is generally much safer than poultry.