It's likely just that they haven't done the legwork to properly support it yet, or don't want to be a whitelabel service. Even the little gas station washes by me push subscriptions now. My agency has built out a few of these services in the past and they aren't cheap, even the off-the-shelf ones.

As an aside, the entire experience is awful. "Pay $25/month and get as many $8 washes as you want!" ...but I only get a car wash every 2 months and since you're tracking my license plate, you already know that.