I would guess because maybe Shim (either Over or Under) Bucket is a more direct approach to opening and closing the valves and thus has less reciprocating parts than would a threaded arrangement, which would necessitate the need for an additional rocker-arm in order to hold the threaded adjusters, which in turn would make the shim-style engine capable of higher RPMs.
Plus, because of the lack of that additional rocker-arm, it would make the Shim designed engine less expensive to manufacture.
