Because MimbleWimble doesn't support Bitcoin script, it supports far less functionality than Bitcoin and isn't a viable upgrade path for Bitcoin itself.
Having said this, it certainly makes sense that MW should be a sidechain, given that huge amounts of Bitcoin's use is for simple "move money from party X to party Y" transactions, which MW does support.
From my understanding, mimblewimble does the most basic transactions in a better and more private way than Bitcoin does. IMO, those are the features we want in the main chain. Privacy as an addon will lead to the very use of it being considered suspicious. I'd rather have privacy as the default and more advanced scripts as the addon.
That said, I do understand how extremely difficult this would be to roll out in a backwards compatible way. Maybe it would be possible to build sidechains on a sidechain, thus with time making the first sidechain the most used chain, while still offering full backwards compatibility by supporting the legacy chain.
Something like this diagram:
Bitcoin
|
Mimblewimble with Lightning Network (Bitcoin 2.0)
| | |
Sidechain 1 Sidechain 2 Sidechain 3
This would let everyone transfer over to the mimblewimble chain in their own pace. People who prefer the old blockchain could keep using that, but overtime, the mimblewimble chain could become the default payment method on the network, while still providing more advanced features and scripts through its various sidechains.