The reason some airfields do not have automatic Met systems is purely down to money, many of these places have basic Met Obs systems that have no automatic facility. Most major airfields have Semi-Automated systems that can either be set to assisted (whereby the various sensors present the data to the ATCO or ATSA who can either confirm it or manually update) or fully auto.
AUTO obs are usually used out-of-hours or when there is not a qualified observer on duty. I believe that at Heathrow there are currently trials to run with fully AUTO obs all the time.
Personally speaking, nothing will replace the Mk1 Homo Sapiens for met obs, the fancy kit still can't see a fog bank nearby or an approaching TS.