This is gonna sound backwards as shyt, but I think its guess of his image/weight.
I agree son can spit, but I think he just aint that marketable to most people because of how he look
I was gonna come in and say this. It's the harsh truth. By "slept on", I assume the OP was speaking on the general public or casual rap fans that probably care way more about an image - the way a rapper looks and dresses - than mic skills. An example would be a dude like A$AP Rocky, who I think is kinda overrated (if I can use that as the opposite of slept on).

