[427 S/O]

Would this have anything to do with the cost?.

Five years ago, magnesium's cost was twice that of aluminum (about $4.00 per kg). This was based on 40,000–60,000-ton expensive, continuous, chemical plants that reduced the combined magnesium in magnesite, carnellite, and seawater to MgCl2; smelters (with their expensive electricity) reduced the chloride to metal. Using thousands of small, low-cost, coal-fired kilns, Chinese producers now directly reduce dolomite in low-vacuum steel tubes and condense magnesium "crowns" in 60-kg batches at one-fortieth the cost per ton of capacity compared with Western producers. From nothing six years ago, China now produces one-half the world's supply and has forced magnesium prices down to nearly those of aluminum ($1.60 per kg).