A lot of the numbers in the 80's are taken by receivers and TE's on the team. You may think that there are less than 10 combined receivers/TE's, so all the guys can fit in the 80's? I think not. Take the Eagles for example. Their receivers are:
There may be a few I forgot, but that list is 9 guys right there. That woudl mean that there would be 1 number left for another receievr in the 80's. You also have to factor in retired players. For the eagles, there are no players with a retired number in the 80's (see below)
Retired Numbers: (6)
15 Steve Van Buren RB 1944-1951
40 Tom Brookshier CB 1953, 1956-1961
44 Pete Retzlaff TE 1956-1966
60 Chuck Bednarik C & LB 1949-1962
70 Al Wistert OT & DT 1943-1953
99 Jerome Brown DT 1987-1997
...but most teams would. In conclusion, it is almost impossible to fit all the receivers in the 80's, especially old teams with more retired players.
Also: Now that you can use 10-19, a lot of receivers are jumping on that bandwagon and getting a new number because it looks cooler. Pretty soon, it may look like there are 4-5 QB's out on the field at a time.