The kamphfer is a shock and awe unit intended for quick surgical strikes get in do some some damage and get out fast, and because it needed speed it's armor was made lighter to facilitate that, also as a lightweight unit it was easy to disassemble and reassemble behind enemy lines and transport cause a few trucks could carry all the parts with little issues while with most standard units for both zeon and the feds you needed a special transport vehicle that could carry their weights and it was harder to hide them in plain sight not very efficient not effective for a sneak attack mission. Plus much like the Spanish inquisition no one expects a wrist mounted gattling gun to just pop out and pop you also 90mm is not a small round (that's roughly a 3.5 inch in diameter bullet for those using freedom measurements) I mean most buildings had a decent amount of tanks can't handle A round of that caliber or size
