Keep in mind Solo takes place a good 8-10 years before the OT. Thats alot of time in between. This thing may have been phased out in that period or its very specialised, or just not cost efficient for mass production.
The other point I’d like to make is the Empire is a galactic wide military industrial complex. Making new and more varied weapons of war is their business. Some end up becoming ubiquitous symbols of the Empire, others end up being not so viable and get phased out or some being more specialised and theatre specific. All these new pre OT designs don’t bother me at all because the Empire is all about creating new weapons of war.