CAT III-B requires RVR50 visibility (may vary slightly per each airline's operating manuals) and visibility often drops even below these minima. Prolonged periods of sub-CAT III-B conditions at major airports worldwide are actually very very rare, and DEL comes to mind as the only one where this is a virtually daily occurance for almost 10-15% of the year.

The resources required to certify aircraft and pilots to CAT III-B are huge. These costs are not something that can easily be justified for a single station in a larger network as long as a viable option to divert/reschedule/cancel exists.