>- Is there any information on why the various zappers
use specific frequencies?

Some, but not all aspects of frequency are discussed at Best Zapper - Confused.

You should also check out the Rife CAFL to see what certain specific frequencies are reported to do.

In general, most frequencies have several effects including specific targeted effects and broad coverage overall effects. That is while any particular frequency will have an effect on a broad range of organisms, it will also specifically have a stronger more direct effect on certain specific organisms.