This is the choice of the developers! It maybe because it is too slow for processing large amounts of image data.
I'm sure it is their choice, but the GKT+ documentation asserts that GDK_INTERP_HYPER is "just as fast and results in higher quality". Gimp has scaling options of Linear, Cubic
and Sinc (Lanczos3)
all of which I have heard about. I have not heard of hyperbolic interpolation
and I'm not about to spring for $90 to buy Wolberg's out of print book (which I probably wouldn't understand anyway). There is scant reference to this method on the web which makes me really
curious. The GDK+ developers will know.
Cubic filtering is missing from the list; hyperbolic interpolation is just as fast and results in higher quality.
Nearest neighbor sampling; this is the fastest and lowest quality mode. Quality is normally unacceptable when scaling down, but may be OK when scaling up.
This is an accurate simulation of the PostScript image operator without any interpolation enabled. Each pixel is rendered as a tiny parallelogram of solid color, the edges of which are implemented with antialiasing. It resembles nearest neighbor for enlargement, and bilinear for reduction.
Best quality/speed balance; use this mode by default. Bilinear interpolation. For enlargement, it is equivalent to point-sampling the ideal bilinear-interpolated image. For reduction, it is equivalent to laying down small tiles and integrating over the coverage area.
This is the slowest and highest quality reconstruction function. It is derived from the hyperbolic filters in Wolberg's "Digital Image Warping", and is formally defined as the hyperbolic-filter sampling the ideal hyperbolic-filter interpolated image (the filter is designed to be idempotent for 1:1 pixel mapping).