etymology - Why don't we say things are pervious?

The pervious/impervious pair makes a nice double-foursome with penetrable/impenetrable, permeable/impermeable, and pregnable/impregnable. All four pairs involve the notion of yielding or refusing to yield to incursion or intrusion, and all register at least some matches on Google Books and its related Ngram.

But though some of these word pairs receive equal respect from dictionary makers, none enjoy truly balanced use in print (and presumably in speech). Here is how the Ngram charts for the years 1700–2005 look for each pair.

For penetrable (blue line) versus impenetrable (red line):

For permeable (blue line) versus impermeable (red line):

For pervious (blue line) versus impervious (red line):

And for pregnable (blue line) versus impregnable (red line):

Of the four pairs, only permeable/impermeable shows anything close to competitive balance. But until 1900 or so, the next-most competitive pair was pervious/impervious, which is surprising in part because pervious doesn't receive as much respect from (for example) the EL&U spelling checker as penetrable does, despite the latter's prolonged weak showing against impenetrable. (All eight words appear in Merriam-Webster's Eleventh Collegiate Dictionary [2003], however, and the Eleventh Collegiate is very far from being an unabridged dictionary.

But the Ngram chart for pervious/impervious also unmistakably shows that people do say that things are pervious. In fact, as this Ngram chart of penetrable (blue line) versus permeable (red line) versus pervious (green line) versus pregnable (yellow line) for the years 1750–2005 shows, they use it more than they use pregnable and about as much as they use penetrable:

The Google Books search results behind the Ngram chart show that pervious is especially widely used in connection with soil and with concrete, asphalt, and other forms of surfacing and pavement.

With regard to why some words that have been in use for decades suddenly or gradually fall out of favor, I think language and competing word choices are so complicated that you would be unlikely to isolate the cause even in a particular instance—and as for reaching a generally applicable truth about how words rise and fall, good luck with that...