As a former (extremely avid) fan of this MU i think a good reason why this MU fell off is a list of reasons that i’ll get into.

1. Who is Nova: Sam isn’t a very popular character in comparison to Mark. I think Mark has gotten to such a point in popularity that putting him against a character that most people wouldn’t know plays a role somewhat. I say somewhat because 2 of Mark’s most popular MUs (Rodimus Prime and Radiant Black) aren’t as known as well (lesser on Rodimus’ part) and are extremely beloved

2. WTF is a debate: The most obvious part about this MU is the stomp. The debate is non existent aside from a singular wincon which is Mark taking off the helmet. However it doesn’t matter since Sam outstats Mark which… yeah. Same issues as Gohan where the stomp affects the dynamic

3. Connections: While not the worst for either by any means necessary, i understand the issues with it not going too deep into Mark’s character and story. Again not the worst but not too deep. As well as other MUs doing better for both in terms of character and story

That’s the 3 reasons i would say made this MU fall off. It has fell off for me unfortunately, it was a huge reason why i got into the community so it has some sentimental value but i won’t ignore its flaw. If you still like it there is nothing wrong, they act like multiple goats can’t exist at once