I think the reason it got delayed slighty is because of a few things.
1. Trains delivered Late - When the Coaster was all fully built, I believe the trains were still not on sight and of course, you need to have trains to ride it dont you, Also the ride needs to have testing, you also need the trains for that too. And even when the trains came, they came in parts I believe and then of course Six Flags or Vekoma had to assemble them, put them on the track etc.
2. More to do they they realised - I think this is basiclly true, I simply think they had too much to do than they realised, I believe the Coaster was fully built by the start of April, but even when it was built, they still needed to get the trains, Do any testing, Make any themeing, Build the station, Build the Queue, Build the new Walkway to the coaster etc. Also dont forget they were relocating the road next to the coaster too.
Hopefully X-Flight is up by May 19th, that means it should be open when I visit the park in June