Motorized Excavator (8043) set alternatives
I'm afraid there is no other excavator in LEGO Technic line like 8043. It is quite unique in that it has both driving and its functions motorized. Which is why it keeps its value even on second hand market. In case your collection consist of more than 42006 set you can look into &quot;bricklinking&quot; 8043 set for yourself. That is, look into buying missing bricks to build 8043 set of your own collection. In fact, owning just 42006 set means you already have 45.3% of 8043.
If you are fine with fan designed LEGO model I can suggest looking at excavators on Rebrickable. All models have instructions (paid or free).
Answer to why LEGO doesn't keep same type of model for a long period of time is quite simple, I think. If you got excavator released two years ago (which is the usual life span for Technic models), you don't really need a new one. You'd probably want to get dump truck, super car, plane or some other model that you don't already have. And if you already got all the types, and LEGO keeps making sets of the same type you'd probably won't spend money on newer models, which is something LEGO doesn't want to happen.