first principles reason is software architecture. sseparation of concerns.
smart contracts are a privatr/federated database feature. security of a settlement ledger is separate and bitcoin makes one thst is universal by its novel and singular security - you can't make a second one. any competitor with equal features will be eaten by the biggest. this is a thermodynamic rule, combined with the singularity of the need for whst bitcoin does - irrevocable, immutable record. for the single most important ledger for a market. it replaces the central bank.
mixing other functions into it is not simply purity but engineering. every distributed system can do 2 of 3 things. smart contracts break the immutability and the base layer (central bank) function.
