Difference between Numpy and Numpy-MKL?
Numpy and scipy rely on lower level fortran libraries such as BLAS and lapack to perform many of their functions. These libraries come in many different versions, including the highly portable ones available from netlib, and other which are optimised for particular systems. In particular, Intel has one called "MKL" - Intel Math Kernel Library. So numpy-mkl just means a version of numpy compiled against the MKL fortran library.
Probably whichever version of numpy you previously had installed was somehow broken, and couldn't find the libraries it needed.