They won't work correctly with other keys.
Keyboards have limitations on which multiple keys can be pressed and how many at once.
Using the arrow keys would interfere with other keys and would have un-desirable outputs.
Example:
I have A assigned to the spacebar and B assigned to the B key while emulating.
Pressing the B key with the right and up arrow keys and spacebar would work as intended, while pressing the B key with the left and up arrow keys and spacebar would cancel out the spacebar. (It happens even with one key, too. You can omit "B Key" from this example)
That is a problem, because Wario would not be able to do a leftwards high jump, or be able to swim quickly diagonally up-left or anything else you would use the A and/or B buttons with left and up.
Mapping the ASWD keys to up, down, left and right (or any other 4 keys, these are the most common keys that people would use for these purposes) would eliminate this problem. Using the number pad would work too, as for it does not have these limitations.
Using the right and up arrow keys together with other keys works fine, but using the left and up arrow keys together will prevent other keys from doing anything at all. This is a hardware limitation that some keyboards use (Not all of them do this though, mostly older ones), and cannot be changed. The term for this is called "Rollover", or in this case, "two-key rollover".
For more info, see this Wikipedia article:
Rollover (key) - Wikipedia
This is why the arrow keys are not suitable for this hack.