Because it requires alterations to how the zords operate.
Not every megazord is a simple Go-Lion combination, so when zords like the Thunderzords or (Possibly) Astro Megazord pop up, it seems like a pointless inclusion or, possibly, hindrance due too the need of extra parts.

I don't really care since the zords I'm interested in are basic in combination (Ninja Megazord, you shall be mine~), but it can be annoying depending on who it's being applied to (For example, the poorly engineered ZB ports in the Dragonzord makes the Dragonzord Battle Mode's legs splay out a lot farther than most people would like).

Usually ZB gets put in the shoulders at least. And that flaw with the Dragonzord was less its fault and more the 2010 Megs' fault.

A fault caused by ZB :p

I'm thinking more for the Thunder Megazord and Tigerzord where you have to rip pieces off the robots to accommodate the ZB gimmick... How long before something breaks and you can't replace it?

If they are targeting the Legacy line for adults, WTF are they needlessly adding ZB ports to these robots?

ZB for the MAIN line (the ones geared for kids) is fine... But not on my $200 Thunder Megazord...