The reasoning i think is that it takes so much more effort and work to compose a piece of "classical" music. You have to have a significant understanding of the tonality of the instruments you use, the capabilities of each instrument, as well as the interactions between the instruments. Beyond that, writing orchestral pieces or arranging for an entire symphony is an extremely difficult task because you have to write music for, say, 50 different people. Of course some parts are the same, but it's a daunting task. Did you also ever wonder why most classical composers don't really come to be until they are older? Something tells me the learning curve is a bit longer and flatter than it is for, say, metal.
Another reason for this idea that classical is "better" is that there are no production "tricks" which make it interesting. The general sonic qualities of every piece are quite similar, and have been for hundreds of years. In popular music, the "sound" is often as much a part of the songwriting, and technology/production is always changing and growing, and often times that is what keeps the music interesting. Something that "sounds" new, often intrigues us enough for that to be the reason we initially start listening to it. Classical doesn't have this "cheap trick" appeal.
One other thing i think can be said is that so much of what is out there is borrowed from classical, or was already done in classical first. Sure, not the "sound" of pieces of music, but if you were to examine the tonality and progressions of a piece of music from nearly any other style, you'll see it's been done long before in the world of classical. There are exceptions (i don't remember ever hearing "bluesy" classical) but in general this tends to be the case.
I'm not really an expert on the subject, but i think there are a lot of valid reasons people have for holding classical in higher regard than most other music. It takes a lot more long-term dedication to really learn how to compose that way, and to me that in itself is an indication that those who can are more talented than your average 20-something metal musician.
