Maybe there's something simple here that I am missing, maybe some of the regulars here can fill in the gaps:

First, 3 things to keep in mind: Please do not try to turn this thread into a debate about these 3 realities.

A)  The BTS collateral system inspires confidence that if a real-world asset revalues significantly, the corresponding bitAsset should maintain its market peg by increasing or decreasing the balance of BTS that that asset corresponds to.  We all know this of course as it is what validates the bitshares platform as a worthwhile cryptocurency.

B) The key vulnerability in the bitShares/bitAssets ecosystem has always been a significant drop in the value of the underlying asset (BTS), this is a very real possibility in an ecosystem with such a relatively small market cap.

C) In the event that the BTS market cap drops bellow it's tipping point, bitAsset shorts would be called and bitAsset holders would realize a significant increase in their underlying BTS balance - unfortunately these BTS would be worth much less. The asset holders would be forced into a large net long position in a now collapsing BTS; furthermore, confidence in the system would be shattered by the rapid calling of shorts and users rebalancing their portfolios out of a newly created BTS long.  The flooding of BTS previously locked up in collateral into the market with no influx of external capital (real world assets) would cause significant further valuation drops in BTS and more and more market peg would be broken.  Again, this is a known limitation/vulnerability that we all accept for the opportunity of investment.

My question is:

Why is the additional layer of abstraction in a more vulnerable cryptocurrency necessary in the first place? As far as I can tell, it only adds an unnecessary level of risk.

Real-world asset=BTC=BTS=bitAsset. 
All of these must retain value to ensure a transition from bitAsset to real-world asset or a gateway will not be able to function.

1) We have a bitAsset, this is "guaranteed" to represent a value-equivalent basket of BTS via held collateral.  OK
2) BTS must retain its value for the reason stated above. ~ Vulnerability
3) For BTS to retain its value BTC must as well.  Without BTC there are no BTS - no one is dealing with banking restrictions to convert asses into BTS.  This is however, largely safe as the size of the BTC market-cap makes it tamper-resistant to all but the largest (ie sovereign) players.

While the BTS market is impressive, the underlying altcoin exposes users to much more valuation risk.
Is there no way to build the bitshares market with 3x collateral directly onto BTC? Maybe through a decentralized sidechain? 

The downside to BTC denominated user issed assets like colored coins is that as an asset holder you trade risk 2 above for risk 1 - ie the issuer can  fail to acknowledge the value of the asset.
I haven't looked into it but my basic understanding of NXT is that it combines both of these risks: faith in user issued  assets (risk 1) and faith in underlying currency (risk 2).  But I don't care to understand NXT at this time so I don't care.

I would think a platform could exist that tracks ownership of digital assets on its own chain (using DPOS - I forgot that is also an important innovation to bitshares) and interfaces with the BTC blockchain to provide collateral to those assets.  The interest generate by shorting those assets could provide incentive to the delegates maintaining the chain.

There's probably something basic I am missing, maybe someone more familiar with the code knows?

Just my 0.02 bitUSD