There are multiple theories on this. One is that UPPERCASE is easier for the computer to process. Another is that nintendo felt that UPPERCASE names were more proper, such as you UPPERCASE a persons first letter of their name. Also, UPPERCASE might be easier for the computer to write out, since it needs to be all in the same caps for faster typing. Finally, nintendo might have thought that UPPERCASE would discern the pokemons name from the rest of the numbers/letters on the screen.