Why did it take five years to "figure out" how to use astrometric calibration sources to deblur LOFAR images? Quanta Magazine's The Hidden Magnetic Universe Begins to Come Into View is a fascinating review of a rapidly evolving field in astronomy.
It contains some statements:

In their paper last year, van Weeren and 28 co-authors inferred the presence of a magnetic field in the filament between galaxy clusters Abell 399 and Abell 401 from the way the field redirects high-speed electrons and other charged particles passing through it. As their paths twist in the field, these charged particles release faint “synchrotron radiation.”
The synchrotron signal is strongest at low radio frequencies, making it ripe for detection by LOFAR, an array of 20,000 low-frequency radio antennas spread across Europe.
The team actually gathered data from the filament back in 2014 during a single eight-hour stretch, but the data sat waiting as the radio astronomy community spent years figuring out how to improve the calibration of LOFAR’s measurements. Earth’s atmosphere refracts radio waves that pass through it, so LOFAR views the cosmos as if from the bottom of a swimming pool. The researchers solved the problem by tracking the wobble of “beacons” in the sky — radio emitters with precisely known locations — and correcting for this wobble to deblur all the data. When they applied the deblurring algorithm to data from the filament, they saw the glow of synchrotron emissions right away.

The filament between galaxy clusters Abell 399 and Abell 401 is also discussed in How is it determined that the X-ray and radio intensity come from a magnetic field bridge between two clusters of galaxies?
Question: Why exactly did it take five years for the radio astronomy community to &quot;figure out&quot; how to improve  the calibration these LOFAR measurements and apply deblurring algorithms? Answers to

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suggest that correcting for spatial variations in water vapor or other effects is a known thing in radio astronomy. However discussions at How large does refraction become in radioastronomy? suggest that the ionosphere becomes increasingly important at low frequency (the &quot;LOF&quot; in LOFAR) so perhaps this was part of the challenge?
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.