Answer:
It has to do with voicing assimilation (voiced sounds vs, unvoiced sounds at the end of the word stem.) before the suffix is added.
Explanation:
Ex....
'P' is a voiced consonant (with your hand in front of your mouth, you feel a puff of air on your hand), so it's phonologically easier (more natural) to say a      '-t' sound immediately afterward.
'N,' in contrast has no puff of air, so it's a voiceless sound.
