That’s because before the TSLPatcher (And even quite awhile after it was released) the only way to use .mod files was to copy them to the modules folder. So two mods with .mod files would be incompatible because you’d either use one mod’s .mod or the other, but you couldn’t use both.
Now the TSLPatcher can patch files inside .mod files, so it’s not an issue anymore. The exception is that some mod authors don’t properly utilize the TSLPatcher’s features, and still just use it to overwrite .mod files in the modules folder instead of patching them.
The Community Patches patch .mod files properly though, so they’re compatible with just about anything.