FWIW most of BA's Shortfall Airbus fleet is MLS equipped (but not the 744/777s, not sure about their 787s).
As a result of the above LHR offers published MLS approaches down to CAT 3b no DH limits (typically 75 metres RVR) for all runways ....so I'm not sure where this idea that MLS is already a victim of GPS or that it has been overtaken already by RNAV approaches has come from. As I understand it MLS is still technically more capable than RNAV/GPS when it comes to genuine low vis ops, but I'd agree that unlike GPS/RNAV it's a technology that's not going anywhere.
As for the "why?" and the original question:as I recall it during the introduction MLS it was being pushed as having lots of advantages verses an ILS: for example curved approaches to low minima - which in reality was not going to happen at many airports e.g. due noise routings; OTOH it provides reduced spacing down the approach- which I believe (an ATCer no doubt will confirm or deny) is a facility LHR ATC use to their advantage to improve the flow rate during Low Vis ops - but whilst it's all good stuff as I understand it unlike GPS/RNAV MLS basically doesn't provide enough commercial advantages 24/7 to have make it a "killer app" vs ILS.
Last edited by wiggy; 21st Jan 2017 at 09:14.