It's because in most cases, it's shoehorned in.
There are thousands of links you could make with any topic. But in many cases, they aren't really all that relevant to look at in detail unless you have a very special interest.
I'll give you an example. It's all the range now for organizations that have been around for a long time to do "work" to explore part links to slavery and colonialism. My university did this a few years ago, and made a big deal about it.
What they found is exactally what you would expect. Some of the original governors had investments in areas that touched on slavery. One owned a ship that transported products to and from the West Indies, which undoubtedly involved slave labour. One had lived in the US for a time and had a servant who had been purchased as a slave. Some of the money donated to the university during this period had undoubtedly come from businesses that in some way involved slavery.
All of this was pretty much inevitable, because the institution existed at a time when that was what the economy looked like, and people who were part of the institution operated in the larger economy.
It's a bit as if today we decided to explore the relation of a universities relationship to Big Business, or the pharmaceutical industry, or even the Apple Company, because those things are morally problematic. Well, of course a university, and the people who are part of it, are all going to have ties to all of these things.
The economy of earlier times, the links of the UK to various nations that were part of the Empire, can be really interesting things. For people interested in history it's good to have a general sense too of how the economics of the period worked. But every painting, or person, or object, or place, does not need to be attached explicitly to whatever the topic of the moment is when it is presented to the public.
Or to put it another way, we don't generally Roman sites or Viking sites in this kind of reductive way, even though they had plenty of slavery within their economic systems.