The simple reason is that you don't fix accounting compliance matters in the currency, you put those fixes in accounting software.
The accounting software stores all the additional fields that are important to it as a part of the transactions.
Then the small business quickbooks software, can send a payment to the larger company SAP software (which has to contend with multi-jurisdictional compliance issues) which further issues payroll in 11 different countries.
The fact that there isn't a currency that does GAAP compliance on its own may be because GAAP rules change, and currency is not meant to change, it is meant to be stable.
The surest way to get a project to fail, is to try to make that project do things that it ought not be doing.
Attaching message to payment is about convenience, not about GAAP compliance. Is bitcoin success depending on NOT FOLLOWING any of GAAP rules?
No, but it might depend on ignoring them on occasion.
The currency remains the wrong place to put this convenience.
Bitcoin AND ALL CURRENCIES are not the place to solve this problem.
The right places could be in a bitcoin wallet, the accounting software, or in the financial service provider, or in any of the volatile entities that rise and fall with the constantly changing regulatory environments in the thousands of jurisdictions worldwide.
This is not an example of a design flaw, but one of design success. By not attempting to resolve problems outside the domain of the currency, the currency is more resilient and stable.
It also provides for the industrious Bitcoin Wallet programmers to localize wallet software that can accommodate the peculiar regulatory issues of the different constituencies. The competitive software market can decide how to send these additional messages to each other through bitmessage, or through some other channel that satisfies the standards.
Another issue which can get complicated is the "unit of account" field. Again it is a complicated issue for a multi-jurisdictional currency. And it is best solved in the accounting software, not in the currency.
To make the point a more clear, GAAP may not be the prevailing standard for much longer even in the US as there is an increasing tendency to adopt the "principle-based" system of IFRS over the "rule-based" GAAP. If compliance needed to be incorporated into the money, such a change would be less feasible. If the US had to burn and reprint all the dollars to comply with a different standard, that would be a mess indeed.
There are many pieces to an economic ecosystem, resolving issues in the proper places helps to keep it tidy, what the OP is suggesting is a way to make it messier rather than cleaner, but it IS a problem to be addressed, just needs to be done in the right places.