Because the green bearing, if installed correctly in a straight housing is just as good as a tapered bearing without needing to bother with adjustments. They are simple and they work.
Someone erroneously made the Ford comment. Ford.....and others have been using the "green" style bearings since at least the 1950s and in some really heavy duty applications. Even though I prefer the original style tapered roller, there's nothing wrong with the green style bearings just because they are that style.
