When monotheism first became an idea it was really different and in some cases shocking to what they people at the time believed, they believed in many gods for different things, so the fact that some people believed in only one God was surprising but also in a way seemed like a good deal to some as they only had to focus on 1 supreme being. As we also know, religions with only one God seem to still be around and be stronger than those with many. (sometimes, not always.)