I remember hearing that the original plan for Big Show was that he was going to be billed as "the son of Andre The Giant". Let that sink in for a moment.
Show is rare for a big man that he actually is very good on the mike. He can play things for laughs, or be menancing.
I think part of the problem, which no-one has touched on here, is his constant face/heel changes. I think that they brought him in as a heel, and soon after, turned him face.
It soon seemed that a Big Show turn became an annual event.
This also tells me that they kept changing their plans for him. Part of this was that he got injured, and also carried too much weight.
I think that he also had a lazy attitude. Show signing with WWE was a big deal at the time, as it was Vince stealing a WCW superstar back (as WCW were stealing away many WWF stars) and being tall always appeals to Vince's "big man" fetish. So he signed him to a ten-year, million dollar plus a year contract. When you have that attention paid to you, it is easy to take it for granted. Show probably thought that he would have everything handed to him.
Also, his Wrestlemania winning record is very poor, and when you have it being made a storyline of, when he feuded with Cody Rhodes, and when you have non-wrestlers like Akebono and Floyd Mayweather beat you at Wrestlemania, it doesn't make you look like a star. He didn't even get on the card at WMX8, but was interviewing at the "WWE New York" restaurant instead.
I think the fact that Big Show was not in the Royal Rumble speaks volumes about where he currently sits in the pecking order.