hindos: Because people are fucking stupid and the minecraft community is also severely autistic:
The best modpack ever made (not counting the Yogbox) comes out, and people go "OMFG WHAT IS THIS THE MODDERS ARE MAYBE POSSIBLY MAKING 2 CENTS LESS PER YEAR THIS IS HORRIBLE".
TMI stagnates for months (even now it's exactly the same as the version for Minecraft 1.0), someone makes a new mod from scratch called NEI which is substantially superior and just quite amazing, and people go "OMG IT LOOKS SIMILAR IT MUST BE COPYING ALL OF TMI CODE BURN IN HELL".
Eloraam makes an awesome mod with unique features that resemble stuff in Buildcraft and Industrialcraft (but are either much better or very different), and people accuse her of "copying" BC and IC.
I'm sure there's a bunch more idiocy that's just not as obvious/public, too. Basically, there is no reason, people are dumb, etc.