While the telescope's individual components meet their requirements, contractor Northrop Grumman needs more time to test them, integrate them together and do environmental testing. In order to monitor the telescope's schedule, NASA is creating a Independent Review Board (IRB) to monitor this testing and NASA will take its recommendations into account when determining a specific launch window. That will occur sometime this summer.
Many suspected this announcement was coming after a report from the US Government Accountability Office earlier this month. The GAO found that ongoing technical issues with the telescope meant that launch delays were likely, and that the project was at risk of breaching the $8 billion cap set by Congress, which would mean it would need to be reauthorized. The telescope has already encountered delays, and it's safe to say that more will follow. It's an incredibly complex, detailed and delicate device, after all.