In a market that is very saturated with feature-rich services, I want to do something narrow, and address a specific need. It makes it easier for me to rely on certificate transparency, as I'm adding new hosts automatically, without risking noisy notifications. I wouldn't want an uptime service to glob whatever hosts it finds in my domain just to tell me they go down, or don't respond to HTTP. But an SSL cert is either served, and has an upcoming expiration date, or isn't served - in which case I'll never alert about it :)