There's many reasons why; his look, he is a faceless artist, we don't know him personally, he's not shoved in our faces and he's a watered down version of many older male pop stars.
I don't think he is a flop. He has had a decent number of hits because he is generic.
Personally, I think he is terrible. Too much auto-tune and saying his name.
He's boring. His music is boring. He's not cute so I can't crush on him. He can't sing. He can't dance without looking like he just learned a few steps. Besides his body, he's useless and can sell a few singles here and there.
And as someone else pointed out, he reminds me a bit of Chris musically and with his dance moves..
However, Make It Up As We Go was one of my favorite songs last year...
Quite frankly I think it has to do with the fact that he doesn't do any features/have any featured artists on his songs. I think people underestimate the power of features or being featured. His promo is also ****.
HIs music is no less bland than Bruno Mars, but Bruno is out working with Snoop Dogg and B.o.B, as well as performing at every award show.