Redundancy

Why did Lucas feel the new to stick in "Young" when he already has the title ending with "Chronicles"?

...because Indy is young in them?

Marketing!

...had to be that ass Rick Macallum

Probably to head off all the "Huh? Who is this? Wheres Indiana Jones?" complaints. 'The Indiana Jones Chronicles' sounds too connected to the films for what is at best a loose spinoff.

It's simply the character at an earlier age / time frame. Calling it just "The Indiana Jones Chronicles" would've given the impression it took place in the same time frame as the movies. Including "Young" in the title clarifies things, and subtly helps denote the difference in tone and style the TV series has from the movies.

I don't know what's supposed to be redundant between the words "young" and "chronicles." They mean different things.