GMforPowergamers
Legend
I have no clue why else they would have done it.
Well no one can say why for sure (well Hasbro accountants maybe) but it is possible:
1) 4e bombed and no one knew it
2) 4e was a huge success just not a big enough one
3) Some one pitched "Hey what if we could double our income"
4) The idea's started running thin and someone pitched a new direction
5) Something none of us thought of
If I were a betting man it would be that 4e 'only' sold X when they wanted Y where X would be a huge enough number that any other company (Say Onex path, or Piazo, or Goodmen) would call it a major success.