Why is not Bech32 address selected as default in Bitcoin Core 0.16.0?
Bech32 Addresses is still not supported on the other platforms. There are many libraries even considering bech32 address as "invalid." 

If I'm using Bech32 address and asking someone who is using another platform to send me some bitcoin they may never be able to make the payment. 

We need some time so it becomes supported everywhere.

Right now Blockchain.info, Electrum doesn't support bech32 address and they're highly used.