Okay I'll be honest - it's because they honestly don't care

I'm sure your friends are happy for you, but it sort of ends there.
I wasn't always overweight. I was effortlessly thin until college. Weight problems never crossed my mind, I never understood why people were fat (boy did I learn), never saw it as a big deal, it just wasn't an interesting subject to me. I tuned it out.
Think of it as any major lifestyle change - ie. getting into a workout program, a new boyfriend, a new car, a new dog, etc. you bring it up once, your friends are happy for you, but then it kind of ends there. If it's not a mutually interesting subject then that's that.
Remember the first friend to have a baby? That's alllll she talks about?
When a lot of people start their weight loss journey, they cannot focus on anything else and cannot stop talking about it (this is no different than smoking, religion, knitting, whatever!!!) - and I think sometimes, in the height of our excitement, we forget this.
After 8 years, I would have driven friends insane, so I come here.