|I think there is several factors that play a role.|
First of all, telnet and by extension MU*s come from consoles, which have a black blackground and electrons bombarded at the stuff that needs to light up, e.g. text. As such, black backgrounds have become the defacto standard. In comparison, word processors were that way in the beginning, and over time they changed - WordPerfect on DOS gave it a blue background with white letters by default. In the end, white became the background because people intend to print on white paper, and so it makes sense. And to double-back upon the original subject of MU* output, that is not meant to be printed.
Second, the ANSI color protocol, which is the old and most-used protocol for colors. It maps actual colors which you can see in the ANSI Colour screen of the World Configuration. Colors are very finicky for as far being readable goes, with contrast and brightness differing per screen and precise settings, and the quality of ones eyesight. To make it worse, once you start turning things around by making the (default) background white, all colours need to be re-mapped to colours that are still legible for the same combinations the original colours were. (And then one also needs to consider all the other backgrounds, making for an even worse hell.)
Finally, many games will have an item like 'a yellow ball' be printed in yellow, either for the whole text or just the word 'yellow'. But once you have a white background, 'yellow' tends to become unreadable, so it is substituted for a brownish shade. (Similar examples work for cyan, lime, etc.) You might argue that the original issue exists for the black backgrounds with 'black pebbles' or 'dark blue trousers', but those will usually have had no color codes associated, or a more readable one. Most people use black backgrounds on MU* games, so that is what is designed for and tested with.
I forgot to write about this other thing. Another factor, but this time from the players perspective.
Calling a light background 'easier to read' is subjective as hell. What works well for you does not work for others. I don't blame you for finding it easier to read, but it is worth noting light-on-dark text is far from a pariah.
Let me back it up with a fact. I know someone who is visually impaired and who relies on Windows features that gives him better accessibility. Among those is the High Contrast theme, which makes everything black, selected stuff green, and text whitish. Other colours are barely used. The screen is most readable to him like that, he can't properly use it otherwise. Other people with other issues or habits regarding their vision might have other reasons to hate white background for long-term reading. I myself find that it exhausts my eyes because it bombards my eyes with direct light from the screen - black backgrounds play far nicer with eye fatigue in comparison.