The video, as others have said, is all that really needs to be said. And the song was not commercial enough.
How was it not commercial? It is catchy, upbeat, danceable. I don't think the song was the problem tbh, as it had more hit potential and was more radio-friendly than You and I and TEOG.
One of my favorite songs and videos -- but I always thought it was a horrible single choice, even before we knew it was going to flop. The only way it would've ever worked was as the lead single with all the hype behind it. It's not catchy enough in the typical way, the wedding sounds might come off as a bit creepy to some, and it doesn't have a sweet melody like TEOG (the only true radio hit of the era for a reason) or Hair (cheesy, but probably would've done well/better). I LOVE the video, but that killed it even more. Non-stans already find little replay value in ~8 min videos (besides Telephone), so 14 min was never going to work.
On a more serious note, I thought MTN should've stayed an album track since day one, literally day one.
My first thought: this is cute, but it's definitely an album gem! And they go right ahead and release it Stupid as hell.