Plastic frames generally cost less than $5.00 to make. They do not require expensive machining to final tolerances, either. So, they are much more profitable for the gun companies. That is MOST of the reason for their existence.
The all-metal guns which are still in production are increasingly being moved to off-shore production (to decrease labor costs). The remaining domestically-produced all-metal guns are becoming the top end of the market, price wise, at a rapid pace.
A thoroughly unfortunate trend, in my opinion....but no doubt inevitable, given the cost/ profit difference.