Different paint.
Different road surface (to which the paint was applied).
Greater traffic volume.
Changed traffic profile using the road (more HGVs, for example).
Heavier vehicles (average car weight - even discounting EVs, which didn't exist until relatively recently - has increased markedly).
More rain.
Hotter spells, more frequently.
Less frequent maintenance / replacement.
There are probably many more possible causes / contributory factors.
They should use the same type of paint they use for yellow lines, they never seem to fade as much, though if they do they repaint them quickly, so it must be much cheaper.