ODT is unfamiliar to many recipients and does not always open cleanly in Word. HTML opens in any browser and can be published on the web as-is. ODT is at home in documents in open-source office suites; HTML is the usual choice for web pages and emails.
The HTML is made inert: scripts, event handlers and forms are removed, remote pictures become plain links and nothing in it is fetched when you open it. The text is read through its structure (headings, paragraphs, lists, tables and images); the per-run styling OpenDocument carries is dropped, so fonts and page design are not reproduced.