Whenever my 94-year-old mother-in-law describes someone as peppy, I know she thinks that person is really swell. Peppiness is her highest form of praise.*
When's the last time you heard someone under the age of 90 mentioning pep? I think it's a generational thing.
So it's no surprise that a lot of advertising from the 1940s promises more pep. Have a look.
*On the other hand, if my nonagenarian mother-in-law describes you as brassy, you're definitely not on her hit parade.