According to this GNU Project page, regarding Microsoft Public License (Ms-PL):
This is a free software license; it has a copyleft that is not strong, but incompatible with the GNU GPL.
They do not explain why it is incompatible. I cannot find any satisfactory answers nor explanations via Google.
Best Answer
As far as I can see, the incompatibility between GPL and Ms-PL stems from the fact that the Ms-PL does not seem to allow sub-licensing or dual-licensing.
As the GPL requires that all code in a GPL-licensed product is covered by the GPL and the Ms-PL seems to require code that is Ms-PL licensed to remain covered by the Ms-PL and only the Ms-PL, this makes it impossible to have code covered by these two licenses in one product, thus rendering the licenses incompatible.
My understanding of the incompatibility comes from this quote from the Ms-PL license (emphasis mine):