Relational DB's resemble ledgers from a business perspective. Most business apps and nearly all financial apps think in terms of ledgers of transactions. Go one to two degrees of separation from a financial transaction in any web app these days and the users of that system will want to line up other activity/transactions in the app with the financial ledger. Then they want to ask questions and have reports on things like: how many users for that customer? how many purchases this month for that customer? what did they purchase? etc.
I think its historical and historical also implies the vast majority of systems aren't likely to change if there is some-other-tech that may be easier to work with. Its a safer decision to pick well known tech because you'll know the trade-offs, support issues, hiring parameters, etc. from the history of all that came before you. To reverse that point - picking some esoteric language/db solution increases the risk of a product/project failing, not because of the technology but all the other factors that surround that technology.