From our perspective, most people start (or at least should start) with the premise that email is not secure. There are things that we can do to make email more secure, but as an email service provider, we have to balance delivery with security. Most customers would rather have their email delivered to an inbox than us only deliver to an inbox where we can validate the TLS certificate presented by the server. In-fact, delivery is the entire reason companies like Mailgun exist.
That doesn't mean we don't care about security. We've rolled out opportunistic TLS about a year ago, we're working on rolling out mandatory TLS soon (with strict certificate verification), and have a slew of security feature coming down the pipeline that I think (some) customers will be really excited about. However, when it comes to delivery, the truth is that features like mandatory TLS are not really requested or demanded by customers. In the email world, delivery trumps security which is not surprising when you work from the premise that it is not secure to begin with.