A number of reasons there:
1 - Slightly less faff to cock and load and so more time shooting
2 - I prefer the fibre optic sights in low light conditions, say up in my loft range where I mostly use it, and they suit
MY eyesight. I know that they are not everyone's cup of tea
However, in saying that, they are both good pistols and both worth keeping. I feel the HW75 is rather well built and has more heft to it than the HW40, but it is also noticeable that the HW75 stands quite high in the hand when on aim.