Oracle positioned OC4J, than WebLogic as the "strategic" application server. With the acquisition of Sun, Oracle also acquired GlassFish. GlassFish is well adopted, open source and came with commercial support. The open source and commercially supported GlassFish products were effectively identical. GlassFish was and is a reference implementation for Java EE. The combination of Reference Implementation (and so well documented cutting edge technology) and commercial support made GlassFish the "Killer Application Server".
GlassFish was popular among developers and was used in most Java EE 5 and Java EE 6 green field projects. Also many projects migrated from flagship application servers directly to GlassFish.
In fact many startups and projects used GlassFish as their main server. Surprisingly, GlassFish commercial support was not very well advertised (and pushed by the presales) and so unsurprisingly the commercial support was also not widely requested by the customers.
In 2013 year Oracle cancelled the commercial support for GlassFish, but GlassFish still remains at the same time reference implementation and is going to be actively developed.fast, is easy and uncomplicated to install, and the deployments are quick. WLP compares to WebSphere, as GlassFish compared to WebLogic. WLP is neither opensource nor Reference Implementation for Java EE, however it is commercially supported by IBM. In fact you can either choose the fully loaded WebSphere, or WLP. WLP already partially supports Java EE 7.
Seems like Oracle and IBM swapped their strategies. Oracle focusses on WLS, and IBM offers both solutions at the same time, as Oracle did before.