Wrong as usual. About me and the draft. How many rounds were there in 1960? 20 rounds..... Number of guys chosen in the draft 240. So there were 12 teams. It went to 17 rounds in 1967 because the addition of more teams even the fewer rounds resulted in 445 picks.
In 1975 the draft contracted to 12 rounds and the number of players constricted to 335. In 1993 it went to 8 rounds and the players taken constricted to 224. In 1994 it went to 7 rounds and has stayed there since.
As for why they went to 7 rounds, think salary cap which began with the 1993 Collective Bargaining agreement. UDFAs were added to fill rosters and were part of the concessions for free agency.
Technology has also improved greatly. It took 100 times longer to collect film reel and scout players. Now coaches can punch in a players name and see every down he played in a row. Drafting used to be much more of a hit or miss.