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.

IBM focussed on WebSphere as the strategic server and the lesser known Community Edition (effectively Geronimo) was meant as an entry level "developer" edition.

Interestingly IBM also withdraw recently the commercial support for WebSphere CE and Geronimo, but at the same time introduced commercially supported WLP.

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.