Simply because you can't create a unbreakable connection between bitcoin price and whatever token you end up with. This is the same flaw that the tokens such as "wrapped bitcoin" suffer from, for example when you swap 1BTC for 1 WBTC there is nothing guaranteeing that in the future you'd be able to swap it back or even at the same rate you may be forced to dump your 1 WBTC for only 0.001BTC.
I think that Bitgo company is vouching (for now) that price peg for Bitcoin will stay the same, but what happens if in near future Bitgo goes bust or broke?
Peg is lost and WBTC tokens becomes worthless, and reading latest Gavin Andresen shitposting, I am thinking that someone is planning just that, moving of Bitcoin to other ''better'' chains with purpose of slowly killing it.
Regardless, Bitcoin has a very limited ability to implement wrapped tokens and derivatives on-chain, but it should be possible to develop side chains with those features.
Sidechains already exist, but they never become more popular, even if some are trying hard (read Liquid by Blockstream)
Good resource for that is Jameson Lopp website:
https://www.lopp.net/bitcoin-information/other-layers.html#sidechains