It was a bit too ambitious for them at the time, there were a lot of changes in the development team and changes in leadership that ended up causing them to eventually change direction.
1.9 has been in the works for quite a while. The hard work behind getting it ready has been complete for a while (responsive default theme, proper template engine based on Twig, etc), but they've still been trying to get it release ready and it's been unclear what the progress has been for years. The features are also a bit limited by today's standards, IIRC alerts still weren't on the 1.9 feature list (which is something almost every forum has nowadays). A responsive default design, however, will go a long way to modernizing it (and will help theme developers immensely, who will have less work to do to create modernized themes).
MyBB's saving grace, in my opinion, is that the plugin community is vibrant. There are a lot of very neat plugins that developers have made that help to overcome some of its shortcomings, and the software itself is extremely customizable. It might still be a little bare-bones out of the box, but it can be used to create some pretty creative and fully-featured forums for those willing to tinker with things a bit.