Why did it take five years to "figure out" how to use astrometric calibration sources to deblur LOFAR images?
The point of the paper is not that no calibration has been used previously. It points out that they now use a more refined method and set of parameters to calibrate the data.
That in general is usual procedure: the more data you gather, and the longer a mission runs, the better you understand your instrumentation and all external factors which influence the data you gather - and especially the sensitivity towards different systematic and instrument errors. Thus with data becoming available you only have the people to work on the problems, often Bachelor, Master or PhD students. Thus prior to instrument availability and detailed data analysis you can only do a coarse calibration based on general knowledge of the problem. Five years is a reasonable and good time interval two have one to two generation of students work on it and get an good overall picture which considers all aspects you mention (and probably more) in the necessary detail and assess their contribution in a way which satisfies scientific scrutiny including the review process.