Well, mainly because they're Starfleet, and Starfleet implies a certain amount of conformity. You'll notice that in canon once the various members join the Federation that different design philosophy goes out the window in favor of a uniform design. Member states can get away with nontrad stuff when it's not in Starfleet, private transports or inactive reserves or cultural enclaves or what have you.
Mr. Fnord interdimensional man of mystery
FenWiki - Your One-Stop Shop for Fenspace Information
"I. Drink. Your. NERDRAGE!"
Mr. Fnord interdimensional man of mystery
FenWiki - Your One-Stop Shop for Fenspace Information
"I. Drink. Your. NERDRAGE!"