Rosters are in. Why are teams only allowed to bring 21 players for the U20 WC?

Games are May 30-June 20th

SA team - Arg, Brazil, Colombia and URuguay
Europe - Germany, Hungary, Portugal, Serbia, Austria and Ukraine
NA - US, Mex, Honduras, Panama ...

Don't think the other regions have a shot

Do big euro teams not take these seriously? No Spain is surprising they are usually a power house at this level. Italy, France..... England? WTF are they