Allah’ represents purity of Islamic monotheism
The other thing, which I consider also relevant, is that the term Allah, in Arabic, is not subject to plurality. For example, in English you can say God and you can also say gods. In Arabic there is nothing that is equivalent to [the English term] Gods, nothing whatsoever. In other words, there is no Allahs for example. This emphasizes the purity of Islamic monotheism.
A third reason, which is quite interesting as well, the term Allah does not lend itself to any gender. In other words, there is no female or male gender for the term Allah. In English you can have god and goddess. In Arabic, this simply doesn’t exist, which shows that the term Allah is a lot more accurate than using the term God even if you are using a capital G. At least it is relatively more accurate in conveying the true nature of the Supreme Creator.