Possibly because of the role that image has come to play in music.In todays music world the image of the artist has become as important,if not more important,than any qualities that the music itself might have. There is definitely a snobbishness in many circles about music,not just between the different forms but even within the forms themselves.
I don`t think that too many people would really question Elvis` musical abilities if it were based on the music,but unfortunately there is also an intricate web of images and prejudices to get past, usually based on the passing moods and likes of the times.
I sometimes think that Elvis` versatility and broad tastes worked against him.... Rock`n`roll purists think he sold out and became too middle-of-the-road...even though he was a great Rock`n`roll artist..... Blues purists don`t want to acknowledge his place in Blues music...even though he breathed it as easily as air and made some great blues recordings... and so on.... I think also that a lot of people don`t want to acknowledge Elvis simply because he was such a huge star and such a huge success....he became a benchmark for other entertainers.