[toni-a]

In the past we had a lot of offerings targeting prosumers, especially by third party, we had plenty of lenses in the 200-300$ niche.
When I got 30D Sigma was offering 17-70, 18-125 along with many others and even a cheap alternative to the kit lens,  now such offerings are very few, almost all new offering are big and heavy lenses, a cheap lens is now 500+ EUR.
The new trend is also for full frame APS-C users are mostly forgotten. all offerings now are ages old, am I the only one complaining

[Brightcolours]

The title promises an answer to why. Turns out the sentence is wrong, and "why is nobody interested in prosumer lenses anymore?" Grammar does matter.

And to your question, the answers are obvious.
What you call "prosumer" is actually not prosumer at all, it is beginner/snapshooter lens stuff. Nothing "pro" about these at all.
And they (those 3rd party lens companies) were not selling much of those lenses at all. Canon started to offer better (AF, IS) lenses, also for low prices, and the market just is not there anymore.
Also, those beginners/snapshooters stay with they smartphone nowadays. And then there is the indent small cheap mirrorless cameras have made is the beginner/snapshooter part of the consumer market.

Did you buy that 17-70, 18-125 when you got that 30D? No. So why are you complaining?

[toni-a]

Without entering into grammatical discussion in a photography forum, about a language that is not the native language for both of us, in Sony APS-C world there's almost nothing in this class, the only lenses affordable are the kit lens and 50-210f4.5-6.3 which are rather poor performers.
Give me a Sigma 18-125 FE and I will get it
Hope Tokina will fill this gap in Sony lineup, for canon and Nikon they are available although old designed lenses

[Klaus]

In the past we had a lot of offerings targeting prosumers, especially by third party, we had plenty of lenses in the 200-300$ niche.
When I got 30D Sigma was offering 17-70, 18-125 along with many others and even a cheap alternative to the kit lens,  now such offerings are very few, almost all new offering are big and heavy lenses, a cheap lens is now 500+ EUR.
The new trend is also for full frame APS-C users are mostly forgotten. all offerings now are ages old, am I the only one complaining

I wouldn't call the  17-70, 18-125 "prosumer" lenses but entry level really.
It is a lot of money but price-wise I'd say that this category is in the 600-1200USD range.

And I think there are lots of lenses in that category.

Entry-level lenses other than kit zooms are sort of dying though, yes. 
I'd like to test e.g. more APS-C stuff for Canon but - well - there isn't much that we haven't tested on the 50D.

[toni-a]

Entry-level lenses other than kit zooms are sort of dying though, yes. 

Exactly and that's a pity, I bought my wife Sony A6000, now she's interested in photography and it's great when we go out together and take pictures, by today's standards 600$ is a normal price for a lens