JMS was apparently holding out for Trek (why he thought he'd be offered I can't know, seems pretty arrogant)
		
		
	 
He wasn't "holding out" for 
Trek.  Word was that Paramount was putting 
Trek on ice for a period after 
Enterprise, then a rumor surfaced that they had changed their minds and were going to do a new series - and were looking for a new show-runner, fast.  Hence his rare public appeal for fan support.  He had had an informal conversation with Paramount about the on-spec plan to "reboot" the 
Trek universe that he had co-written.  (Paramount never saw the actual outline, deeming it "politically awkward" for them to do so at that time.)  
Paramount had considered 
B5 after all, back before Warner Bros. bought it, and there are conflicting accounts of their either asking JMS to do a 
Trek series instead of 
B5 or even adapting his series idea to set it in the 
Trek universe.  (Some say that they later did that themselves without JMS's hlep. 

) 
So publicly suggesting himself for the job under those circumstances wasn't especially arrogant.  As a long-time 
Trek fan, JMS just couldn't resist going for it when an opportunity seemed to be there, even though he 
already had a number of other projects on this plate.  With 
Trek off the table for a couple of years (meaning nobody else is going to get it, either) he was free to take this fall 2006 project when it was offered. 
Regards, 
Joe