quick reading through wikipedia gave some context, but nothing specific to why an egg. i think the reasoning is simply that egg is more commonly known than a cocom. which i know somewhat sounds ridiculous but im thinking that based on that egg is a very common food ingredient and commonly seen by most people (sometimes daily)* (*atleast like in developed places i'd assume). while seeing a cocom is gonna be rare to uncommon for most people at least to the scale of eggs.

secondly, with metaphors already existing like 'coming out of your shell'. an egg would socially make more sense (easier to connect to the metaphors that already exist). so egg cracking -> coming out of your shell would be easier to presume and understand the reference then something unfamiliar or relatively uncommon. that is not to say that the metaphor of caterpillar to a butterfly isn't common, but egg checks a lot of other 'common' checks here, so stastically would be used as the main metaphor more commonly. (though to note im making up what counts as important boxes and statistics here, along with what the stats are according to what i know and not any real data to link; so who knows u know)

thirdly, egg could've just been the luckier metaphor