John4321
Because for getting on 18 months many of us who do this type of work (Engineers), are currently stood down/redundant/unemployed/left the industry so not as many manhours available to do the work.
Also, mods like this tend to get done on a planned check in the hangar. If the aircraft is not flying as much and using hours it isn't reaching the hours limit for the check, so checks are getting further apart on a calendar basis. Also, if the aircraft had been put into parking/storage mode at some point, usually no maintenance is carried out apart from those requirements under the parking check regime.
Having said that, if it is a simple mod, no reason it can't be done on the line with a bit of planning, which is probably what will happen now if this is the root cause!