Because they don't: the manufacturer (SE) releases it when it meets Fido specifications. Fido merely sells phones with personalized software just like any other provider around the world. SE can't update and release every single version of the o/s at the same time.
When Fido is happy with the customization they will give SE permission to release. After that it is up to SE to fit it into their schedule. SE has to spread the demand on their upgrade servers to prevent overloads (entire world, remember). Over the air upgrades are dangerous because of the possibility of data corruption and battery failure. So, just keep checking SE site.