A few reasons:
1) Why do people still play the lottery? There's something about gambling that is addicting to many people. Thus, keys are higher in value
2) As people have mentioned, the other better things that you can get from a crate also raises the overall value of the key in comparison to the crate.
3) Some stranges are just more popular than others, even if we could ONLY get stranges from crates, the more commonly used and popular weapons would still be worth more than a key while the unpopular ones would be worth less.