why do HTML Comments in the WYSIWYG break the page display?

This is a duplicate of another question and (I’m sorry!) a bug in WP. As @Otto says in the comments to that question, this is an edge case problem (not many users know or care about using comments) and is not likely to get fixed soon unless you’d be interested in contributing.

EDIT:

I looked for more information on this in trac, and found a few related bugs describing exactly this problem. There is a workaround suggested:

Just before the close comment tag, put another open comment tag. That second open comment tag will be commented out, but it forces the close tag to be recognized and not be reformatted as an en-dash.

Example that fails: <!-- <b> text </b> -->

Example that works: <!-- <b> text </b> <!-- -->

In other words, if you have comments that enclose other tags, just make sure the last tag is another open comment tag.

Hope this helps! (Better late than never!)