12-10-2020, 07:06 PM
I think it is because in Dutch, the digraph [ij] represents one vowel (it is a diphthong in standard Dutch, but that is still one vowel). As such, it is often written and the ligature of [ij] is thought of as a single unit. This probably facilitated the Middle Dutch custom of swapping it out for the similar looking (and in Dutch otherwise unused) [y].

