It looks like they used to have a couple of his books there. See: http://www.accesstoinsight.org/news/news9612.html
Perhaps the people at Suan Mokkh wanted to host those.
Remember that Access to Insight is just one web site, which features lots of translations, books, and articles by Thanissaro Bhikkhu, plus a selection of other articles and books from various authors.
There isn't much by a large number of other very prominent figures. Only three books/articles by Mahasi Sayadaw, nothing by U Pandita, nothing by Goenka, one by Ven Gunaratanara. But, as with Ajahn Buddhadasa, there are plenty of resources elsewhere on those people...