A lot of people who inspect the source seem to say things to the tune of ‘bitcoin doesn’t have smart contracts for security reasons’ which seems a worrying statement...
I am kinda illiterate in it so no comments.
But there's still a reason why bitcoin doesn't have it and it's not even a BIP.
Ok I am not a specialist, but I have read some about this.
Ethereum is known to be "Turing complete". His means that anything programmable can be programmed in the Network. There are some mechanisms which protect the network against attacks such as DOS (denial of service, like applications that would be looping forever and crash the network), like the gas system.
However, as anything can be done, it's both a strength and a weakness. As someone one day may be able to make a smartc ontract that would somehow attack the network.
Another important characteristic of ethereum network are the Dapps. Decentralized Applications are applications that cannot be stopped. This may also be a security problem...
Bitcoinon the other hand is not Turing complete and it's programmable languages are much simpler. So the attacker would have less possibilities to attempt something.
There are many other people here that know a lot more, I would like to hear them