Propably because a sufficiently large power bloc within these countries was pushing for socialism at the time of creation of their constitutions. Portugal, for example, was prevented from declaring a Peoples Republic by NATO outright sending warships to the capital during their revolution.

Reactionaries then took over in these states

India & Sri Lanka were both in the Non-Aligned Movement and had good ties with USSR/Eastern Bloc. India was the first country where a Communist Party was elected in a bourgeois democracy system. So, Communists had significant influence on the state even if the state itself was aligned mainly with the bourgeoisie. Communism/Socialism wasn’t considered a no-no word in these countries unlike the West.

and see

and also see this article on the resistance Israel faced trying to build ties with India