Because they're better cars than the Leaf and thus maintain resale value better. The Leaf isn't coming out on top in this comparison. There's a reason some things are worth more than others.
Friend of mine is a Leaf enthusiast and even he concedes it's pretty awful. It could have been great, but the batteries degrade so fast you can practically watch the range decrease with every drive.
And you're quite off on the price of a Model 3. I think you're confusing it for the Model S.