The simple reason is that there is no necessity to call water (H2O) by any other name. The main purpose of giving a name to a thing is to provide it with a separate identity. No other substance is known to have the same name 'water'. read more
Water is the normal oxide of hydrogen and it is known by this common name for a long, long time. The only other known oxide of hydrogen is called hydrogen peroxide (H2O2). So, there is no confusion or problem in continuing to call the normal oxide by its common name ‘water’. read more
Water is a name that predates the use of the words hydrogen and oxygen. It has been kept for historical reasons. Another example would be using ammonia rather than nitrogen trihydride. BTW, other possibilities for alternate names for water would be hydrogen hydroxide and dihydrogen monoxide. read more