Reasons are many:
I. While the TMC has been electorally routed by means fair and (largely) foul, that's far from the end of the story. The organisation is in complete disarray right at this moment.
A merger would only add to the confusions.
II. Despite being devastated, Mamata, apparently, still remains quite popular with large sections of the Hindu and, particularly, Muslim subaltern masses.
The defectors have no credibility.
So, she should be encouraged to re-establish her claim as the owner of the party. Not abandon her claim and lose her mass appeal in the process.
Let's also not forget that much of her popularity is anchored in her perceived image as the champion of Bengali identity and, hence: "Jai Bangla!" She must not throw it away.
III. The Congress in the state is too weak to digest mercurial and highly ambitious Mamata.
IV. Together with her very formidable popularity, she also carries a huge baggage of negativity. In the case of a merger, that'd be passed on to the Congress. Its credibility would be seriously undermined.
V. A close working alliance, instead, to oppose the BJP tyranny would make far greater sense.
(Despite the electoral, and subsequent, debacle, she remains the most suitable face to lead a mass resistance against the already underway barrage of multidimensional onslaughts unleashed by the double engine government in Bengal. She is to be actively encouraged.)
That'd, perhaps, also force the Ram-Bam to do a rethink. And if that comes to pass, the resistance would be significantly strengthened. In fact, may even trigger a spike.
VI. The fate of the battle in Bengal, including the one against detention, aka "holding", centres would have a tremendous bearing on the battle on the national scale.