Why? What is so magic about the 100 year mark? Does the simple turning of a calendar page impart significance? Why is a wreck that is 99 years old not of importance while one that is 100 years old is?
The 100 year mark is a lazy mans number and is always a moving target, pick a year and live with it. The real fact is that very few ship built since 1880 are of archaeological significance, historical significance may be another matter. The USS Arizona is of great historical significance to the US (perhaps not as much to Japan) but not all that much archaeological significance. But on 7 December 2041 it will have no magic increase in historical or archaeological importance than it has today.
That's just the way it is. As of this April, the Titanic will have been down for 100 years, so she becomes fair game in the eyes of cultural heritage.