PAST TENSES OF VERBS

If it's blow, blew, blown
And know, knew, known,
And grow, grew, grown,
And throw, threw, thrown,

Then why is it not
Mow, mew, mown,
Or sow, sew, sown
Or snow, snew, snown
Or tow, tew, town?

It's a wonder anyone can learn English as a second language!