For all its good and bad points, the M14 was one of the shortest lived US military rifles ever. It hung around officially for years but was really only issued on a large scale from 1960 or so till 1966. Im amazed we can still get USGI parts and mags as it is. The FAL and G3 on the other hand both were used for long periods of time and very widely distributed. The G3 spans from 1959 to roughly 1995 in Bundeswehr service, longer in the reserves. It is all a matter of supply and demand. I suppose the real question is "why doesnt someone manufacture decent m14 magazines at a reasonable price?"
As for the M14 being currently issued. It is totally a stopgap measure. Its an indication of the military not being prepared logistically. If we need a 7.62 rifle, we should have had one in the procurement and supply system. Right now the M14 is being used by select members of some units (although this is being somewhat overblown in the gun press). The problem is the M14 is not supported by the supply/maintenance system. When these guns break (and they do), the Army or individual units have to find parts on the commercial market. That is no way to support a weapons system.
After 24 years of military service, including two Bundeswehr tours and a tour in Afghanistan, Ive been personally exposed to these systems and issues.