just takes time, many ota customers are cheap and wouldn't pay for two tuners, but they'll eventually add it. when pip was in regular crts it was over priced which drove down adoption and consumer comfort level with said tech. pip was sorta added to crt tv's after the crt had plateaued technologically, they had no where else to go to add premium features. flat panels just keep getting better so adding that additional level is just not as high a priority.
also with tivo and such its not as necessary. you can get a dvr for over the air broadcasts these days as well, thinkn its the dtv pal or somethin.
pip would be an additional feature to this and so less desirable.