Supply and demand. Would a full time extra day of NFL games water down the product? Part of the allure of the NFL is its limited availability. Baseball and NBA are not urgent watch games. You can turn on a game any day of the week for long periods of time. It is more urgent to watch the NFL because it is only on 2 days a week most of the season and for only so many weeks a year.
Baseball and NBA games are not urgent watch games since each team plays 162 and 82 games, respectively. Individually, each game's impact to the post season implications is pretty insignificant. With only 16 games per team, each individual game in the NFL is much more significant. This is why I will go out of my way to watch every regular season game for a team I follow in the NFL, but won't turn on an NBA game until March.