My knowledge about BU and in-depth knowledge about Bitcoin are both very limited and may be wrong. But here is my understanding of why BU's solution can't be included in Bitcoin:
The blockchain of Bitcoin is statelessly verifiable, while the blockchain of BU is not.
Being statelessly verifiable means given any number of versions of blockchains, one call always tell which one wins, under any circumstance, and everyone else will agree.
In Bitcoin, the verification of blocks (hence the blockchain) doesn't involve any local or temporary state. Everything needed for verification is on the chain. However, local states are used for verification in BU. Because of that, there may be chances that the chain is forked, since different nodes may have different local states. Another possible issue is that when a new node joins the network, it may not be able to decide which version of the blockchains is the right one when it's given more than one versions, either genuine ones because of the forking or man-made ones provided by attackers.
Besides the aforementioned issues, which might be impossible due to my misunderstanding, there may be unpredictable issues.
I think this is a design philosophy difference. It's too big for Bitcoin to accept.
Could any Bitcoin expert/master please confirm if I'm right?