I know, what I meant is, "Why did they release such a modbreaking update?"
I later found out that what was modbreaking was actually the Terraria update which basically changed most of the coding, and not tModLoader
The reason many (almost all) mods broke with this TModLoader update is because there were many changes to vanilla Terraria in 1.3.5. Because of that the TModLoader devs needed to chance much of the code to work with the new version. That in turn means that mods need to change their code to work with the new TModLoader 0.10. There really was no way for them to make it so previous mods would automatically work with 0.10.
Edit: I just noticed that you found all this out already. Sorry about that.