If certain cruise lines (i.e. Carnival) wanted everyone in formal wear on formal night they would REQUIRE it instead of SUGGEST it. Carnival attracts a wide range of age groups (especially many in their 20's and 30's). Younger generations are more casual and place less importance on dressing up. Very soon this generation will be the majority that is cruising. Society in general is getting more casual. Here on the West Coast it is very common to see people in very nice jean or very nice bermuda shorts in your nicer restaurants in San Diego, Los Angeles and San Francisco. No one bats an eye.
I enjoy the Mexican Riviera cruises, as they attrack more west cost folks than those from middle America or the east coast. The folks out here are more laid back and have a more live-and-let-live attitude.