Why do people refer to God in different names?

The english-language Word "God" probably derives from an old root Word meaning "The one we call upon."

If that's so (as is believed by the majority of liguists) In a sense, when we say "god" it's a sort of placeholder Word - a description of that being, as opposed to the being's personal name, and the placeholder means, basically "the one we pray to" or "The one that is prayed to," or "The one that we ask for stuff."

Peoples, throughout history, have often been defined in terms of "who it is that they pray to." Hebrews, in a sense, are often defined by being "the people that pray to the God of the Hebrews."

Christian are sometimes defined as "people who worship Christ." (I personally tend to prefer the ones who try to follow the example of Christ, as they see it, with less focus on the praying.)

But basically, the Word "God" means "The one that is prayed to."

People are probably more likely to capitalize the G when it's _their_ god, and to not do so when they're talking about gods they don't believe in, worship, or pray to.

Zeus is a god, because Zeus is prayed to by some people (yes, even today). YHVH, Jehovah, "I Am that I am" and Jesus can be called god, or gods, in the sense of "People pray to them."

In short, "God" isn't a name - it's a title. It's a description.

God is called God because the name "god" means supreme, very powerful, and God is that.