The main reason is that it is hard to implement this feature correctly at this moment. It is possible that we will implement it in a future API upgrade but unfortunately I'm not able to give any ETA.
It depends on your use case but wouldn't be possible to work around this using page-break-inside:avoid? This is just an idea, but I'm thinking of something like this:
Let's say you have #div-1 and #div-2:
If you want to avoid a page break before the #div-2 element you could change the code like this:
Would this help?