Why aren't existing customers eligible for the New National Internet Plan?

I've been using Xfinity internet and three mobile devices for the past two years.

Now that the discount on my internet plan has ended, I'm paying $136 per month for 600 Mbps.

I've called them several times and the "best offer" they gave me was $71.xx/month for a downgraded speed of 300Mbps.

Is the "New National Internet Plan" not applicable to existing customers?

Why is Xfinity ignoring its more valuable and loyal customers?