You don't want to calibrate against something interesting, because of you see something crazy, you'll never really be all that sure it isn't because your instrument is goofy.

Astronomy has long had an issue with this. Back in the late 19th century when the magnitude system was being formalized, an astronomer named Norman Pogson chose Vega to calibrate the system because it is easy to observe in the northern hemisphere. In this system, which came to be the most dominant magnitude system, Vega by definition has a magnitude of 0 in any filter.

There turned out to be two issues with this choice. The first is that Vega has a very unusual spectrum for a star. This means that more normal stars appear to have weird differences in their magnitudes between different colors. But it's not the stars themselves that are weird, it's just a weird choice of zero points!

A more serious issue became apparent when CCDs became more common in the 1970s and 80s. It turns out that Vega is somewhat variable. You can define the zero point of the magnitude system to be the average brightness over a long period of time, but that doesn't really help you on any given night since the equipment needs to be calibrated daily (or more frequently --- temperature and atmospheric changes require re-calibration).

Another source of annoyance here is that Vega is also very bright. This was a benefit in the days of photographic plates. But modern telescopes with CCDs cannot observe such a bright star. It almost immediately. So this makes calibrating the equipment trickier. (You essentially need a two step process where you use a small, specialized device to calibrate against Vega and then measure the flux from a dimmer reference star, and then measure the reference star with your telescope.)

Imagine if we decided to calibrate against a currently-assumed nondescript stable, in field of view star (HD 84406) but later it turned out was part of a strange stellar phenomena that we couldn't forsee (and didn't have the science for) until later on.

I guess we can't win :)

Or the beings in that star system build up a Dyson Sphere around it causing it to change drastically from our vantage point.

"The Stars Are Going Out And Here's Why That's A Good Thing."

It may be that they don't want to use too bright a star because it would saturate their detectors, so perhaps they picked dim, but not too dim, in the right direction.

Edit: the link posted by muds while I was writing this gives the explanation.

There's a great explanation of this on the astronomy stack exchange (https://astronomy.stackexchange.com/a/48317).

Summary:

- Available for observation for a prolonged time

- A star that has just entered its field of view

- Don't want a star in a field that is too crowded

- The star should be bright, but probably not too bright

On the plus side, we'll get the best view of HD84406 that we've ever seen.