Because it is pretty hard to configure compared to other SMTP providers. People who need help selecting what mail service to use are not likely to be able to figure out how to get SES to work.
Lightsail is easier than EC2, but Digital Ocean is easier still.
I imagine that it works just great for those who can make it work. A novice can probably figure out Mailgun in 15-60 minutes. SES is likely to take 4X that. It’s designed for professionals, especially professionals with tools to automate it.
Which didn’t make sense to @lakshya: