[g_orbital]

why does isomer Z prefered in simple wittig reaction?

[Dan]

The unstabilised Wittig reaction is under kinetic control and so proceeds via the lowest energy transition state.

There is a good explanation in Clayden, Greeves, Warren & Wothers, p816.

Note that the stabilised Wittig is under thermodynamic control, and so gives the more stable E product, also covered in the aforementioned text.