Simple cost, size, and weight reduction. 75% + of the people can`t see the difference anyway. When they want to get real cheap, they use mirrors instead of a prism.

If you look at my D3 & D700, the D3 has 1:1 and it has an obviously larger pentaprism to accomodate. Same with my F2 ( 1:1) and FE2.

Katz eye can not make a focus screen for the D3 because it is slightly larger than than the D700 which is the size limit blank they can make. The D3 and D700 are both full frame digi cameras for those who do not know.