It's just that the 1987-1993 stang were very popular and there aren't that many 1979s runnin around. There is just not enough market for them to bother making something JUST for the 1979. It would simply cost them too much money to make it.
BTW 1981 is my favorite year model
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THE SLO PONY
1992 Ford Mustand LX. 2.3 liter four-banger, auto. Future mods: V-8 swap.

Fly it loud, Fly it proud!!!
If it ain't broke, you ain't tryin hard enough