My understanding of rootstock is that it is a turing-complete language (like ethereum), with which to run smart contracts off bitcoin. I do not know exactly how they do it, but the idea is that bitcoins are locked (or transferred into another account), equivalent tokens that have the value of the bitcoins are run on rootstock, when some consensus is agreed upon for the outcome (by some voting mechanism), the bitcoins are released. The exact same thing can be done with ethereum by creating a bitcoin-ethereum 2-way peg. What am I missing?
Nothing, really. But:
Ethereums value is tethered to its usability and its use, while Bitcoin is first and foremost supposed to be a value storage tool.
This is more of a philosophical standpoint than something I can fund with real world facts, but I think, that a base which is as widely used as possible has a lot of big advantages, as opposed to this weird balkanisation we have right now with hundreds of projects with really specialized usabilities, which is kinda like having cash you can only buy very specific products with.
Imagine a scenario in which you have a special money to buy bread, another to buy fruit, one for gas and so on. In addition to the hassle of exchanging one currency for another if you need bread but only have fruit money, they change their exchange rates all the time. That's not what a currency is meant to do; a currency is supposed to be a representation of universal value. Bitcoin tries to be such a thing. Ethereum on the other hand is supposed to be the "fuel" to run smart contracts and so on, so essentially, Ethereum would be "smart contract money" in the above scenario.
By the way, here are some additional thoughts from me regarding a possible project that moves in a similar direction:
https://bitcointalksearch.org/topic/pre-ann-hub-1689947