[youpii]

Three reasons I can think of.

 

1. There is no 85mm f1.8 with AF which renders that smoothly
   
2. 40 x 1.5 = 60mm, 40 x 1.6 = 64mm. One can't get a similar FOV with 50mm or 85mm lenses, not on FF or on APS-C.
   
3. f0.85 x 1.5 = f1.28, 0.85 x 1.6 = f1.36. There are no other 60mm or 64 lenses which will offer such shallow DOF even on FF. Well, of course there is the Nikkot 58mm f1.4 which comes a little close, but that is as expensive a lens, and without any reason. And it is not very sharp.
   

Personally, I am not yet convinced that 40mm on APS-C is a nice focal length to have. So, right now I do not view it as an interesting lens, FOV wise, for me. But who knows, if I actually had a 40mm on APS-C I might discover it is a FOV for me. Its bokeh, that is something interesting for me.

But that is just me, others may find this lens super interesting, and I will respect that. I like how it renders the images.

 

There are a few vintage 55/1.2 & 58/1.2 that offer similar FOV & DOF and are built like rocks.

I have a Minolta MC 58/1.2 that costed me around 300€ and is much smaller.