Because 1440p is (nearly) 2 times the amount of pixels of 1080p.
And 2160p is 4 times the amount of pixels of 1080p, that's why it's called 4K.
It has nothing to do with horizontal or vertical resolution.
This is a common misconception perpetuated by the fact that 4K = 4x 1080p. It suggests that this is the origin of the naming scheme, when in fact it's just a confusing coincidence. The actual origin of the 4K naming scheme comes the (approximate) number of horizontal pixels.
For instance, if you look at the supported resolutions for high-end cameras, you'll find that something like "6K" or "7K" isn't a single specific resolution. They'll list a whole bunch of resolutions matching different aspect rations, and they'll all be "6K" for example whenever the approximate number of horizontal pixels is in the 6 thousand range.
I'll list some commercially available resolutions with their branded "-K" designation here, and you'll notice that using "K" to mean "how many times 1080p" only really applies to a single case. For everything else, it refers to "approximate thousands of horizontal pixels".
4K = 3840 x 2160 (4x 1080p; approximately 4K horizontal pixels)
5K = 5120 x 2880 (7x 1080p; approximately 5K horizontal pixels)
6K = 6016 x 3384 (10x 1080p; approximately 6K horizontal pixels)
Apple XDR example
8K =7680 x 4320 (16x 1080p; approximately 8K horizontal pixels)
This brings us to to 1440p. It makes no sense to call it 2K. If anything, 1080p is 2K (since 1920 x 1080 has approximately 2K horizontal pixels).
1440p (2560 x 1440) would more aptly be a "2.5K" resolution.