Why ISIS adjacency cannot be established when the MTU mismatch between devices？
Because ISO specifies that IS-IS Hellos must be padded, using type 8 Padding TLVs, to within one octet less than this maximum, partly to allow a router to implicitly communicate its MTU to its neighbors. More important, sending Hellos at or near the link MTU is intended to help detect link failure modes in which small PDUs pass but larger PDUs are dropped. If a neighboring interface does not support the minimum required MTU, the padded Hellos are dropped and no adjacency is established.
When you deploy ISIS in network, you must guarantee MTU consistent between two connected interface.