Ork physiology and genetics/ DNA is so robust and hardy that it would still be a difficult task designing said weapon.
No harder than for any other species (i don't see why their genetics would be more robust, they are made up of the same elements and bonds as us), find a disease that greatly effects them and you are well on your way. Then, given their lack of genetic variation, you can basically wipe out every Ork in existence from space.
Oh yeah, I know it isn't impossible, but seeing as they haven't made any success yet it's unlikely they are able to at this time. And the Eldar have had the longest time of any race to develop an anti-Ork weapon.
How often are biological weapons such as this used
at all? It's not the Orkish resilience that is the problem, it's that combat in 40k must remain at a very low level of complexity to accommodate the sort of battles the game represents. My original comment was a bit of a tongue in cheek remark about how technology in 40k is quite laughably basic.