Because of people like you who insist on taking our time harping about old versions.
If you guys kept it private or elseware then sure people could help you.
But you don't, you make mod packs, have issues, or whatever, and then you/your users come to us and waste our time.
Combined with what you just said, all issues that you have for old versions have been asked and answered many times while the version was relevant.
We have a limited manpower thing, so we've decided that it's best to focus on things that actually need fixing, and not 5 year old tech.
We don't care that you like spam clicking your sword instead of waiting on the cooldown. We work on Modern Minecraft.