Most programmers didn't have time to master both Objective-C for making iPhone apps and Java for making Android apps. (This is a good time to note that although the word Java is in JavaScript, they're significantly different programming languages. JavaScript's official name is ECMAScript but JavaScript is easier to type and pronounce. Historically it was kind-of meant to be similarly named for some marketing buzz.)
It was well known that HTML5 worked darn well on both the iPhone and Android devices (and on a lot of the other platforms that were coming out or being revised). PhoneGap was a project put together to allow HTML5 to be used inside an app that could be placed inside app stores. So you didn't have to know nearly as much of Objective-C or Java as you would if you had to make two separate copies of your app (and more and more versions as other platforms become more popular).