Probably because Fire Punch isn't a tutor, and it makes absolutely no sense for Tepig to learn Fire Punch.
Anyway; Gengar and Nasty Plot. Several of its Pokédex entries literally say it does just that, yet no Nasty Plot...
They doesn't explain why they didn't make it a level-up move for Pignite and Emboar.
And Gengar can't learn Nasty Plot? I didn't know that. o_o
Although considered how good it is already, giving it Nasty Plot would probably shoot it right up to Ubers, even though Gengar + Nasty Plot makes perfect sense...