The answer may be found in many text books. A Ka line is produced by an L electron filling a K-shell vacancy - this event is more probable than being filled by an M electron, which produces Kb. So Ka production is more probable than Kb.

See for example Cullity, Elements of X-ray Diffraction, chapter 1 (which is the source I just used)

The intensity is related to mode parameter, the amount of ka line from k to l shell is more probable and hence in maximum quantity than k to m shell.

It is all about probability and X-ray fluorescence efficiency.