Left parties are known to bark, but do not bite. Congress took them lightly & went for the deal. Had they taken BJP into confidence & discussed with them earlier, there is no doubt that BJP would have supported. RESULT: It would give a good impression to both Congress & BJP for successfully signing the deal. Congress didnt want good name to go to BJP
So, they thought that Left, as usual, will oppose intially, but not let down the govt. But, Lefts were taking it seriously & BJP also opposed the deal externally (even though they are for the deal). Bcos, if things goes fine Congress would earn good name, which BJP didnt want. So, they were watching silently how Lefts were barking & when the situation came that Left went to the extent of withdrawing support, BJP pitched in & came in support of Congress. Its bcos, if mid term polls are to come Congress would gain Sympathy votes. Mid term poll at the current moment is not good for BJP.
Ultimately everything that is happening for couple of weeks is a political drama. Only one good result of this drama is that we come to know the real face of Left & they would loose a lot in future.!!!