Because HTML mails are almost always spam anyway.

And disabling HTML should help you avoid tracking pixels which tell the sender if you opened the email or not, etc.

You don't need to disable HTML for that. Thunderbird blocks remote content in HTML by default and only displays images embedded in the email. Isn't the tracking pixel mostly snake oil since it mostly tell if the user use gmail (preloading images) or an email client that block remote content by default?

See my reply to sam_lowry above. Short answer: security.