There is a bit of history and context needed here I think. The last Opera 12 was released about a year ago. Last year they announced they would switch to using Blink instead of their own rendering engine.
The engine switch came with a complete rewrite of their browser as far as I understand it, so they prioritized Windows/OSX stability for a year and now finally came up with a Linux build again. The big gap in version numbers is due to their new browser starting at 15 + the adoption of a much shorter release cycle.
Once Linux builds are made stable I expect they will continue to release builds on all 3 platforms as they were doing for the many years before this slightly extreme shift.