Post WW2 late 40s, the world is divided in blocs based on ideological lines, american capitalists and soviet communists.
Meanwhile our PM Nehru has his own elitist vision of non alignment and dealing through diplomacy in times where india stands b/w crossroads of choosing either. https://twitter.com/vedansh91907775/status/1359457534315171841

Moreover, Congress under the de facto dictatorial hand of MKG is going to plan their nation on Gandhian-socialist line of thought. However , if they were to explicitly mention socialism in the preamble then this newborn country would possibly face enmity with USA

Keeping that in mind, Nehru and committee decides not to add socialist in the preamble since it meant marxist-leninist socialism rather than gandhian socialism during those time to avoid possible conflict with capitalist states by outright making a declaration to the world.

Similarly secularism was identified in the west with the idea of divorce of state and church , whereas the Indian state was willing to interfere with religion and practices but not favour any. Therefore adding secular to the preamble would certainly be a stupid thought.

But muhh indira gandhi 42nd CA 1976 ??
Adding secular and socialist to the preamble doesnt change the structure of constitution necessarily, IG adding those words was INCONSEQUENTIAL, India had always been a secular state , refer to article 25-26-27-28-29-30

India has not only been both 'secular' and socialist since its inception but also grants special rights to linguistic and religious minorities for developing their institutions.
These all clauses were present in the constitution ambedkar made.

moreover morarji desai undid almost everything IG did in 1976 by passing the 44th CA 1978, instead of blaming IG alone, you should look upto each and every other PM who has been in power ever since.

but what do chaddis shout ?

AmBeDkAr CoNsTiTuTiOn GoOd, InDiRa GaNdHi BaD