“In consumer products, 2560 × 1440 (1440p) is sometimes referred to as 2K,[13] but it and similar formats are more traditionally categorized as 2.5K resolutions.”
It'd be pretty weird if it were called 2k. 1080p is in an absolute sense or as a relative "distance" to the next-lowest thousand closer to 2k pixels of width than 4k is to 4k (both are under, of course, but one's under by 80 pixels, one by 160). It's got a much better claim to the label 2k than 1440p does, and arguably a somewhat better claim to 2k than 4k has to 4k.
[EDIT] I mean, of course, 1080p's also not typically called that, yet another resolution is, but labeling 1440p 2k is especially far off.
You are misunderstanding. 1080p, 1440p, 2160p refer to the number of rows of pixels, and those terms come from broadcast television and computing (the p is progressive, vs i for interlaced). 4k, 2k refer to the number of columns of pixels, and those terms come from cinema and visual effects (and originally means 4096 and 2048 pixels wide). That means 1920×1080 is both 2k and 1080p, 2560×1440 is both 2.5k and 1440p, and 3840×2160 is both 4k and 2160p.
> You are misunderstanding. 1080p, 1440p, 2160p refer to the number of rows of pixels
> (the p is progressive, vs i for interlaced)
> 4k, 2k refer to the number of columns of pixels
> 2560×1440 is both 2.5k and 1440p, and 3840×2160 is both 4k and 2160p.
These parts I did not misunderstand.
> and those terms come from cinema and visual effects (and originally means 4096 and 2048 pixels wide)
OK that part I didn't know, or at least had forgotten—which are effectively the same thing, either way.
> 1920×1080 is both 2k and 1080p
Wikipedia suggests that in this particular case (unlike with 4k) application of "2k" to resolutions other than the original cinema resolution (2048x1080) is unusual; moreover, I was responding to a commenter's usage of "2k" as synonymous with "1440p", which seemed especially odd to me.
1920x1080 is 1080p.
It doesn't make a whole lot of sense, but that's how it is.