There could be several reasons. Most likely, the comment you submitted included potentially offensive language or a link to another site (see FAQ on including links below).
The ArcaMax team has attempted to set up a system that allows nearly all comment posts that meet family-friendly guidelines -- but we cannot ensure that our filtering mechanisms are perfect. In other words, if in the rare case your comment is not posted and you're absolutely sure that your comment is family-friendly, it could mean that our system was overly strict. But we would rather be overly strict, than allow offensive language on the ArcaMax site. The editors will monitor questionable comments and approve them as necessary.