I charge for my writing for two reasons:

#1:  While the material is a Gift of Divine Providence, I spend much time & energy on putting it into written form, and 'the workman is worthy of his hire'.

#2: People do not appreciate 'free' things.  This is a psychological truth, unfortunately.  We do not value Wisdom that we do not have to work for.  My writing is of Value, it is well worth reading.  Attach a dollar sign to something and suddenly people understand that fact.