Backpacks use up too much vehicle item cargo space, atleast in my opinion.
Generally, one may consider the vehicles' item spaces too small.

Specifically, when trying to load Mortar Backpacks into a Zamak or HEMTT you will not be able to fit more than three backpacks into this huge truck. While there will probably be special cargo variants in the future, I'd say, that this is just unrealistic and annoying.

This may apply to smaller vehicles aswell, for example the Strider can not take more than 2. In my case I wanted a 4-man-team of two recons and two mortar guys roll out in a strider and the mortars would not fit. Even if the mortar guys carry half of the two mortars, you can load hardly anything else. Though this may be more realistic for a smaller vehicle like the Strider or the Hunter. {F21173} {F21174}