Because the more complex the protocol is the more risks it carries.
By adding all sorts of junk to it is not a good way when it comes to money.
It must be simple and stable, thats all it needs to be succesful.
If you want complexity then use ethereum or other stuff.
That's like saying the internet should only be used for email, and smart phones should only be used to make phone calls.
The problem here is that you are using an expired frame of thought. A frame of thought that has been made redundant due to the invention of Bitcoin and the Blockchain.
Bitcoin is not 'just' money. Bitcoin has record-keeping ability due to the Blockchain. Bitcoin allows users to initiate programmable contracts. Multi-signature transactions are also possible.
Bitcoin is a piece of software that employs human beings to secure it's network (miners) and then pays these human beings via block rewards - which makes bitcoin the first real world implementation of a decentralised autonomous corporation.
Put simply, bitcoin cannot just be put into the box of being "either" a payment network or a settlement layer because it does MULTITUDES more than just those two things.
It has the power to do things that have not even been defined yet
So I think room has to be given to allow bitcoin to continue to mature, instead of looking for small boxes within which to confine it's potential