There are a few reasons but i think it hasnt been legalized because it is a rather new Asset according to the standard of Worldwide adoption. It has been around for 16 years, A long time but it hasnt proved its stability yet as it had many ups and downs in these years. another reason might be POWER, Governments Dont like losing Power and with Bitcoin's Decentralisation they are very likely to lose it as it gives people power in their own hands unlike Fiat Currency.