As an experienced teacher, I would give my (few remaining locks of) hair for a good answer to this. On that basis, I will share my thoughts...
Firstly, students have very little direct experience of it, whereas with reflection and refraction they already have much (mirrors, glass) or at least some (waves of light on the bottom of the pool, pencils in a glass of water appear broken) to guide them. So it is largely theoretical to them.
Secondly, it involves some fairly detailed maths (at A level) and is often taught early on in the course (at A level) before they have much experience of derivations, sine functions etc.
Thirdly, it is counterintuitive that red will diffract most after they have (often) just learned that red light refracts least when dispersed by a prism - they need to use the equation (
Fourthly, (tell me if I’m going on a bit) there is massive confusion over monochromatic light, white light, widths of fringes, spacing of fringes, colours of fringes, visibility of fringes (
Finally, the whole issue of ‘d’ being the number of lines per metre, per mm or per inch confuses. It is probably their first exposure to reciprocal quantities and is often (no, very often) mixed up with the slit spacing, s from a previous (and largely pointless) equation.