Interesting question which to be honest I can't really answer myself... but I'd basically inquire by taking the flip-side of https://www.protondb.com/

Namely gamers like me usually check ProtonDB to see what they can play. Here it would be interesting, and I'm 99.99% sure Valve does that already, to check which games do not work and what's the commonality behind them. It means one can then identify the gaps and try to address them.

Still, to venture an ELI5 answer : games are usually build for Windows. Games are using "bricks" like Lego to avoid re-inventing the wheel. Instead of having a health bar, a game developer might use a "health bar" brick. When you have a collection of such useful bricks, you typically call that a library. That library then makes the work of a game developer much easier but not all libraries are made equal. Some popular libraries target only Windows and thus the bricks make assumptions on how the software running the computer, the operating system, works. So... what Valve does is trying to make new bricks to stack on so that game developers don't even have to use libraries they are not familiar with. They "just" use their typical bricks, Valve "injects" in between their new compatibility bricks and voila, unbeknown to the game developer, their Windows game works on Linux!