Because virtually every alt is traded against btc not against the USD. People use this to hide when Alts have dropped. If btc goes up by 10% and Alt A goes up by 5% against the USS it looks bullish, but in reality it is btc pulling up Alt A and Alt A has actually lost value against btc.
Just holding the btc is always an option (and usually the better option) so comparison with btc makes much more sense.