Ships are like cars, some can be lemons from the day they left the yard. The Franklin Roosevelt had a long history of maintenece problems since she left the yard and is speculated why she, the last of the Midway's, to be decomssioned almost 20 before her two sisters. The maintenence of a ship falls upon the skipper, so some shipe are better taken care of than others. Other ships may have more "milage" and hard work on them. Take the Lexington, after WW2, she didn't see a lot of hard action, from 1945 to 1953 she was laid up, was practically rebuilt, then in 1961, became a training carrier which didn't put a lot of wear and tear on the ship, thus why she was in commission until 1991. Other Essex class ships, like the Oriskany for example, which was a younger carrier, worked non-stop (except for two SBC conversions) from her launching in 1948 through a number of very hard cruises during Vietnam. By 1972, the ship was worn beyond the economic scope to keep it in service, also, she was obsolete. Just like the oil burning carriers (Kitty Hawk and Kennedy) now, they just cost much more to operate that nuclear carriers. On the Constellation website, the skipper explains this very well when asked why the Connie was being decomissioned. He compared the fuel bill of the Connie to that of the Ronald Reagan like that of a SUV to a Geo Metro.
Politics has something to do with it as well. The Enterprise, as compared to the newer nukes, is way past it's economic prime, but too many brass are too close to the ship to allow her to leave the fleet. She has too much public and sentimental value, and they claim to be testing her to see if the new supercarriers can maintain a 50 year service life.
The Lex is a 1/700 Hasagawa Essex kit I conververted back in 1991to her 1956 appearance for a former crew member. I should have some pics ready later in the week.
Scott