Basically because IntelliJ had support for Play Framework projects and was getting weird errors in Netbeans for some reason.
At the time Netbeans seemed ( and was ) dead on the water so I invested a bit o energy and frustration to learn the IntelliJ ins and outs of operating.
Still, I think and maybe it's an unpopular opinion, Netbeans auto-complete was top-notch and more "intelligent" than IntelliJ's, also much lighter IDE, IntelliJ has to thank the silicon Gods for all the cheap available RAM and processing power.