Perhaps it is because the Eocene was much warmer and as it cooled they either moved further south or went extinct.
"Living archaeids are known from Australia, South Africa and Madagascar, a classic Gondwanan distribution. Fossil archaeids, however, are known from the Jurassic of Kazakhstan, China, the Cretaceous of Myanmar, Baltic amber, and Madagascan copal. ..... The presence of archaeids in the eastern Laurasian part of Pangaea in the Mesozoic and western Laurasia in the Cenozoic appears at first to contradict the present-day Gondwanan distribution of the family. Archaeids are small, litter-inhabiting spiders, and their dispersal abilities are probably low. The present disjunct distribution of the family suggests a geological history that pre-dates the break-up of Gondwana in Jurassic times and the fossil occurrences suggests a cosmopolitan distribution in the Mesozoic. ..... Fossil evidence thus demonstrates that the present-day distribution of archaeids probably results from a reduction in the former range of the family, perhaps as a consequence of Neogene climate cooling." - Selden et al. 2008. Palpimanoid spiders from the Jurassic of China.