What is HTML?
An HTML table is what a web page shows and what a browser copies to the clipboard. It carries structure a plain grid cannot - merged cells via colspan and rowspan - which is exactly the part a converter has to expand back into ordinary rows.
What is JSON?
JSON is the shape data arrives in when it comes from an API. Tabular data usually lands as an array of objects, one per row, with the column names repeated as keys in every single one - verbose to read, but unambiguous.