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 NDJSON?
NDJSON is one JSON object per line, and nothing wrapping them. That single change makes it streamable - a program can handle a line at a time instead of holding the whole array in memory - which is why logs, data exports and machine-learning datasets ship in it.