FastMail is using JMAP, a protocol designed to support push notifications. MXRoute is stuck with SMTP/IMAP which don't support those. There might be a different reason, but I think this is probably the core issue here
I think saying they are stuck with standard email protocols is a bit of a stretch. JMAP is not widely implemented outside of Fastmail and certainly isn’t used by Apple Mail, which actually uses a proprietary IMAP extension (XAPPLEPUSHSERVICE).
I sometimes feel the complexity is present by design to increase the switching cost. Once you understand it and set it up on a project, you are locked in, as the perceived cost of moving is too high.
FastMail got access to it and it is not clear how, without any public announcement or documentation.
It is as if Apple just picked a favorite and went with it.