1. you meant the first locally installable software!
2. imho, simply no. google finds 1.5 Million phpBB boards, 1 million ikonboard installations, and a half a million UBB Forums.
it has been the most popular 'cause they where among the first, giving away a free board. and they "invented" the now standart "flat" view, the ubb codes, "["-markup-style.
and hey, look at this pic:
(beside, why not put on a img-ubb-tag? like the url, email, i tags..)