The reason?
I HATE, HATE, HATE complex templating engines (such as Smarty) that rival programming languages in their own right! PHP itself started out as a templeting engine for HTML pages ... now we have templating engines built upon templating engines. Ridiculous.
Anyway, Drupal's theming system at least gives its users the option of standard PHP templates OR a number of other systems. XOOPS is too interwoven and dependent on Smarty.
XOOPS has more cool modules but Drupal seems to be easier to develop modules for.
Other promising projects are Xaraya and eZ Publish, but they too suffer from ridiculously complex templating systems.
I only say these things because I love XOOPS and think it could be improved by eliminating its reliance on Smarty.
Just my 2c.