Because that’s the definition and it’s not used to mean anything else.
The prefix "nu-" is an informal term used in British English to indicate a modern or updated version of something. For example, "nu-metal" music is a term that uses the prefix. The word "nu" originated in the 20th century and comes from the word "new".
Everything after DS9 is NuTrek because it’s the most modern group of Star Trek shows. The movies are not NuTrek because they’re just blockbusters movies. NuTrek is not inherently pejorative or negative. The “Nu” in “NuTrek” is being used the same way it was used in NuMetal (which also wasn’t pejorative. It’s just new and fundamentally different from old trek. Just like NuMetal is newer and fundamentally different from Metal.
Okay, sorry to be pedantic, but you've said DS9 a couple of times and I just, who categorises them like that? Voyager and Enterprise are both after DS9 and I don't think most people would consider them "NuTrek".
Also, this is a pretty silly argument, you're right that "nu" means "new" in English. However, I wonder if it's starting to become a bit like "Modern" in reference to art or architecture. Nu-Metal is actually a pretty old genre these days, and there are newer, more popular ones like Baby Metal.
However, I wonder if it's starting to become a bit like "Modern" in reference to art or architecture
You mean like referencing a specific time period instead of just “recent”? Well sure… but that would be specific to which Nu-thing you’re talking about, not “Nu” itself. Even your example of “Modern” refers to entirely different time periods and time scales depending on whether you’re referring to art or architecture. 1890 was part of the “Modern art” period but “Modern architecture” is wouldn’t exist for another 40 years.
Nu-metal was popularized in the 90’s but Nu-prog wasn’t until the mid 2000’s.
And in all of these cases, the terms were coined with the term NU to mean “new” at the time they were named. Nu-metal is old now, but it was new when they named it that and is why they named it that. So whether you define “NuTrek” as post DS9 or the movies or the current batch of discovery-era shows, you’re still using “Nu” to mean new, recent, or modern, not just in its time period of release but new in that the approach to them has changed with the times.
I don’t think anyone is using “NuTrek” to specifically compare it to NuMetal.