It is simply a matter of path dependency (symbols are older than ISINs) and culture (whenever data is handled by humans, symbols have an advantage: IBM is easier to remember than US4592001014).

But I agree that anyone who handles financial-instruments data should prefer that people use "Unique Instrument Identifiers" (UTIs); even regulators do.

But ISINs are no UTIs: ISINs may not describe a contract uniquely (see e.g. the example in this answer). Also, an ISIN does not contain information about the trading venue. So to describe price data, an ISIN would need to be accompanied by information about the trading place (for which, however, a standard similar to ISINs exists; see https://en.wikipedia.org/wiki/Market_Identifier_Code).

It is simply a matter of path dependency (symbols are older than ISINs) and culture (whenever data is handled by humans, symbols have an advantage: IBM is easier to remember than US4592001014).

But I agree that anyone who handles financial-instruments data should prefer that people use unique instrument identifiers. In particular, regulators do (they typically refer to Unique Product Identifiers).

But ISINs are no unique identifiers: ISINs may not describe a contract uniquely (see e.g. the example in this answer). Also, an ISIN does not contain information about the trading venue. So to describe price data, an ISIN would need to be accompanied by information about the trading place (for which, however, a standard similar to ISINs exists; see https://en.wikipedia.org/wiki/Market_Identifier_Code).