A Director of MSI's Notebook sales Mr Sambora Chern told Fudzilla that so far the company tries to stay away from Nvidia's MXM graphic port. MXM will simply make any notebook a few millimetre thicker and MSI doesn't believe that the end user will benefit from it.
Normally you don't want that your end user cracks a notebook open and upgrade the graphics card as it can easily break it and then send it to RMA. That of course costs you money.
MSI simply picks the graphics card that it believes that will be in demand for that period and integrates it in the motherboard, and there is just enough place even for 512MB graphics memory.
Once MSI decides to change the graphics it has to redesign the notebook motherboard but usually that doesn't take that long and the notebook graphics usually last a bit longer than the desktop parts.