I chose DiCaprio for based on the fact that he's proven he's good at playing the charming character who changes to psycho in an instant. Also he is very expressive - the picture I added was from the revenant showing a facial expression very similar to Darkseid - I could have easily picked a picture from Django etc.
Whoever plays Darkseid will be in prosthetics and CGI'd so to get the best out of the character you need a great actor ...if Paul Giamatti can play Rhino, Ben Kingsley the Mandaring and Jesse Eisenberg Lex Luthor, then Leo can definitely play Darkseid.
Also, while I know it's unlikely, it is worth remembering that...
Tom Hardy
Christian Bale
Heath Ledger
Michael Caine
Jack Nicholson
Morgan Freeman
Jared Leto
Will Smith
Viola Davis
Ben Affleck
Russell Crowe
Gary Oldman
...to name but a few, have all appeared in DC movies - are Pitt and DiCaprio that much bigger than all those names? So why not?