What is the difference between high and low frequency IGBTs?
I find that all the parameters are similar.

Not all of them: Turn-on delay time 21 / 27 ns

There are other differences as well. The differences are small though.

So one is slightly faster than the other.

My guess is that these devices are identical but that they are binned meaning, after production they are measured.

Then the "fast" ones ( Turn-on delay time = around 21 ns) are marked AIKW50N65DF5

and the "slow" ones ( Turn-on delay time = around 27 ns) are marked AIKW50N60CT

Then Infineon can charge a bit more money for the fast ones if they feel the need to do that.

Another possibility is that one device is a slightly improved version of the other device. Often small changes are made in the manufacturing process allowing for a "better" device. Again Infineon could charge a bit more money for the newer, faster ones if they feel the need to do that.

Some customers might have a design-in with the older, slower device and have no need for the newer faster device. These customers often do not even want the newer device as that would mean they would have to re-validate their product again (as something has changed). At Infineon they're not foolish, they want to keep their customers so they keep selling the older device as well.