It's because you can only morally love virtue. Money is morally neutral, much like any other tool. Loving a tool is ascribing virtue to something which not only lacks virtue, but is incapable of virtue. Loving money is the root of evil not because of something evil about money (you might as well say a rock is evil), but because of humans who lie and attribute virtue where none exists. Once you start attributing virtue where none exists, you elevate yourself above God by claiming that your subjective whims are the determiner of good and evil. Which is replicating the original sin in the garden. It's not the fruit, or the dollar bill, or the gold coin that's evil. It's when you claim to know better than God what is good and evil, and to be able to ascribe goodness or evil unilaterally.