JMS has been trying to distil and bottle it unsuccessfully for years. He told an amazing five-year story over B5's first four seasons*, but then each time he goes back to tell more stories in that universe it becomes more and more clear that he can't come up with anything capable of standing next to what he's already done there.
*Context: they crammed the planned s4 and s5 into s4 because they thought they would get cancelled, but then they got unexpectedly renewed so JMS had to write what was effectively an epilogue s5.