To give a very casual explanation, both mains are of the form "do this a bunch of times and return the results". Your first is "do nothing and return the ()s", but importantly, it has to execute all those nothings.

Your second is "print hello a bunch and return the ()s". The list it wants to eventually return gets bigger and bigger as more prints happen, until poof!

You should look at how replicateM works again, and hopefully it will make more sense with that in mind.