Does anyone know if there exist other reasons beside of money, why SJG based their rpg adaptation of Munchkin (their main cash cow currently) on DnD 5.5e instead of, you know, maybe their own well-known rpg?

Regarding the money reason:
Being based on DnD 5.5e means probably a wider audience for Munchkin, but being based on GURPS could lead to introduce GURPS to a wider audience in a cross-product campain.
But I guess SJG has mathed the math and behind this.

I run a Discord Community for PBP games using GURPS.

We teach the system in demos and long discussions as needed. There are no stupid questions. We have several experts in the system and over a dozen games in action at any time. Over 50 active players and hundreds of lurkers watching, curiously. Lurkers can read all active games, ask questions, but not interfere in the actual in-game RP.

We are a friendly community first, as gamers. If you want to join, we’d love to have you. Here’s the link: https://discord.gg/EnE2eJjjh2

Come on in, ask us anything (relevant) and see if there are any interesting games starting up, or pitch one and see if folks are interested.