Twitter’s CEO Dick Costolo Is Stepping Down(techcrunch.com)

  a) He has another company to save which is in itself a
  full time job, given how hot the payments space is.

  b) If Twitter should ultimately (and rather ingloriously)
  be sold to say a Alibaba or Facebook or (longshots)
  Microsoft or Google, Jack wouldn't want to be the guy who 
  'trimmed the fat' and right-sized it for a sale. It wouldn't
  be good for his personal brand. 

  c) Twitter hasn't yet found an identity - even with
  300 MM users (even Costolo hasn't contested this as
  recently as May, in an interview [1]). Jack can only
  breathe life into it, if he makes one hell of a pivot away 
  from the way Twitter is perceived by the masses - way
  too confusing to use for most [2] and lacking an identity
  (People still ask of Twitter what they don't of Facebook - 
  what Twitter will be when it has finally grown up)[3]

  Bonus reason : Twitter has gone through 5 product heads
  in the five years while Costolo was at the helm. A part 
  of me wants to believe that it is not all the fault of 
  Costolo and that Twitter needs a fundamental rethink. 
  I doubt Dorsey is willing to do it. I just don't think 
  he can, unless he gives up his gig at Square.