Probably for the same reason we don’t get paid in bitcoins yet - neither Ada nor Bitcoin are considered stable coins like the us dollar. They could probably accept payment in Ada for some types of transactions, like donations or purchase of apparel… Until someone, e. g. Cardano or another group that’s taking a similar, formal approach whose name at the moment eludes me, figures out how to formalize the definition of a stablecoin, the fiat currencies will remain de facto means of day to day. transactions for most people.
The only thing we have currently in crypto that resembles a stable coin, as far as I am aware, is USDT - tether. It’s pegged on the US dollar, one for one - or so they claim. They achieve stability by pegging it to the dollar which is guided by a set of properties resulting in a stable coin, namely inflation, centralized control over debt based currency creation, backed by your and mine’s promise (under threat of law enforcement if we fail) to repay our debts rather than anything else, and centralized control over the interest rate which controls the supply of moneys lent by the money printers (the Fed), and every bank further down the trough, all marked up, by the time it hits you on your car loan or mortgage.
My guess is any stable coin will have to guarantee these mathematical properties that exist for the USD… whether through staking, the existence of a crypto-Fed guided by stake holders, controlled inflation… or perhaps built in inflation with interest rate control, if it is going to be analogous to fiat.
See, I don’t really think crypto is reinventing human behavior or how finance has been done since the dawn of currencies… what it is doing is making it more accessible and very transparent. With that, increased accountability is bound to ensue, and with it hopefully greater integrity - e. g. no more too big to fail because your uncle is a golf buddy with the chairman of the federal reserve (hypothetically speaking) whom you can get to quantitatively ease your debt obligations until you are ready to repay…
In an ideal world, they would be allowed to fail and out of the ashes of whomever collected on their debt, hopefully a new, more nimble player will arise after they sack the CEO and the entire board or course.
Then again, maybe that’s just a pipe dream, crypto or no crypto… and people are going to remain immutable in their out of integrity behaviors whether on the net in some cryptochain… or in 3d spacetime with fiat. Crypto is no panacea for these orthogonal issues to finance/currency, such as human behavior… but it ought to make it a little bit better than the current state we are in.
Just think about this tidbit… if you could merely account for all dollars in existence, that alone would be a breakthrough…along with how much of it moves and between whom in most cases. It’d be a real boon to policy decisions.