If you go back to the first tall ship race in 1956 you will see the people who organized the race saw it was very popular and formed a new organization. From there the whole tall ship genre has seen quite an evolution but somewhere in this loose classification that now exists was, most likely, profit motive.
Maybe that was some of it, as money is behind lots of motivations, but I think it was also part of motivation was insure the viability of their organization and "grow" it so to speak to increase the participants. Maybe for money, but maybe also to embrace the "old era of sailing" amongst more people. If it remained just these huge ships the organization would just stayed a small finite number. The other other smaller Tall Ships which were part of the era and design of Tall Ships would be shut out or formed their own orgnization, which would have been much larger.
There is a large educational/ experience component in this organization also and it encouraged oppertunities to a large group.
When they have their races between Tall Ships, they came up with the classification system and had some way of comparing like ships with like ships, just as we do today with the modern classifications.
The motivation for the different classes is contained in the this statement
A tall ship is not a strictly defined type of sailing vessel. Most of us use the term to mean a large traditionally rigged sailing vessel, whether or not it is technically a “ship”. The United States Coast Guard’s training ship Eagle, for example, is technically a “barque”. A tall ship can also be a schooner, brigantine, barquentine, brig, ketch, sloop, or a full-rigged ship depending on the number of masts and the cut of the sails.
For the purposes of classification and race rating, Tall Ships America adheres to the descriptions found in the Racing and Sailing Rules and Special Regulations established by Sail Training International.
All square-rigged vessels and all other vessels over 40m (131 feet) length overall (LOA)
Traditional-rigged vessels with a LOA of less than 40m (131 feet) and with a waterline length (LWL) of at least 9.14m (30 feet).
Modern-rigged vessels with a LOA of less than 40m (131 feet) and with a LWL of at least 9.14m (30 feet), not carrying spinnaker-like sails.
Modern-rigged vessels with a LOA of less than 40m (131 feet) and with a LWL of at least 9.14m (30 feet), carrying spinnaker-like sails.