Many reasons. Higher margins. Face value of a pre-'82 cent vs value of the copper is considerably higher than face value of a nickel vs metal value of same.
Also, while the potential value of a nickel right now is about 5.2 cents, it's actual value to anybody else is just 5 cents. The potential value of a copper cent right now is 2.4 cents. However, it is quite easy to sell them for more than 1 cent, and the closer people believe the melt ban to being lifted, the more people will pay for them. It is hard indeed to picture anybody paying more than 5 cents for a nickel right now, particularly since you can still buy them at banks. After the metal composition for the nickel has been changed for 10 years or so, that might change, but it could take even longer.
That being said, I "hoard" nickels prior to 1960, as well as copper cents. I absolutely hoard pre-1982 Canadian nickels, as they are 99.9% nickel. I'm tempted to do so with dimes.