One of the reasons this has been so feasible has been the fact that the engine (MOAI) uses GL ES to manage the framebuffer itself - giving the same look and feel on all platforms the host runs. This has been, honestly, revolutionary in terms of building a single app that runs everywhere.
This now becomes more complicated because the engine has to be modified for Apple platforms to use Metal, and represents another fork/splinter in the unity of the host itself.
I wonder if their decision to use a non-standard graphics API is due to them wanting to make this style of development a lot more difficult in the future - i.e. are Apple passively antagonizing the cross-platform framework builders in order to establish an outlier condition for their platforms? The cynic in me says yes, of course this is what they are doing ..