Simply because sand eats hob filter impellers; not as bad with canister filters, but you'll still be cleaning sand out of one. A friend had 3 Oscars in a 180 gallon tank with hob filters, and had to replace the impellers monthly because the Oscars managed to get sand close enough to the intakes.
hmm...I haven't had any problems with my 2 hobs on my 75 gallon that has a 2 inch sand substrate over an inch of lava rock. then again, I do water changes three times a week and clean my filter out weekly.