Why does the word 'Well-to-do' mean 'rich'?

The expression "well-to-do" has always meant rich. The 1794 use of this expression is:

Ann Low told me that her mother lived in the country, and was very well to do.
_{Old Bailey Proceedings.}

Etymonline uses the Oxford English Dictionary as its source. The 1825 date comes from OED2. It wasn't until 2014 that the page was updated (OED3), adding the 1794 quote (among others).

It's also worth noting two earlier expressions that also mean rich:

• Well to live (earliest attestation: 1568)
• Well to pass (earliest attestation: 1609)

"Well to pass" comes at least in part from Dutch wel te pas (Middle Dutch wel te passe).