Why write water as H2O instead of OH2, HHO, HOH, OHH
It's not written like that because in chemistry there are rules for how you must write chemical formulas. I don't feel like explaining them all here, but a simple google search would help you out. I'd suggest googling something like, "writing chemical formulas" or "writing chemical formulas for molecular bonds (since H2O is molecular)."
Try this. Its as good an explanation as any
Because really, the proper way to say it is "dihydrogen monoxide"
Who cares? If it's in the middle of a complex equation with arrows indicating transfer of electrons/particles, it could be more suitable to write OH2. But generally we simply write H2O by convention.
The form 'H2O' is reasonable. The water's inner structure detemined it.
There is just a rule for it. Follow the rules and everyone is happy.
There can only be one structure that has the molecular formula of H2O, hence we do not need to spell it out like HOH. You must be getting the idea from organic compounds where there are usually more than one structure for one molecular formula (for example C2H5OH and CH3OCH3 are different although they are essentially C2H6O).
Whilst I agree that it depends on how the elements bond as to what compound they actually make up, there is only one way in which H20 can be joined H-O-H
H is less electronegative than O so it goes in front by convention.
Formulas and whatnot are about letting people understand what you're talking about. Even if H2O wouldn't be as technically correct as OH2, it's what people know and since people understand it it's easy to stick to.