Because there is a field in the DD message that describes the mtu value of the router's interface. In the process of establishing an ospf relationship, the DD message generated by one end will be sent to the neighbor, and the neighbor will check it and find that the mtu does not match his own, so he will not continue to interact with each other, and therefore the neighbor's state will always be stuck in the EXSTART state. Just change it to be the same on both ends.

It doesn't matter if the mtu is smaller, as long as it's the same on both ends, it won't affect the final neighbor establishment, it just might take a little slower (but it's not easy to see).