Was there a reason why they did that scene?

probably to A) show that the creative team remembers it, B) remind us that it Big Mac/Marble was a thing, and C) potentially plant the seed for a future plot thread

I find it sadistic cause Marble already has to deal with her dead end life stuck in the rock farm, the least they could’ve done is give her a chance rather then just purposely show this

I didn't say that it wasn't cruel but I do think that it's something that will come back into play if the show goes on long enough and decides to explore it further. Also seeing as they've alluded to the fact the Apples and the Pie's might share a common ancestor Hasbro probably didn't want to risk the implication of having two characters in a possibly incestuous relationship. No matter how many generations removed.

Even their stupid song and dance didn’t distract the insane fans from that episode either

Marble,try harder,next time and win the Colt of your emotions.

i hate you big mac

this was such a great nod to the fans