People talk about coatings and lens/prism materials because they are somewhat measurable. Optical design is a part of it, too, but not so objectively measurable without tools like interferometers. QA and QC are also relevant to the end product.
So just like anything, the quality of the components, design, and construction all play a part. The real difference is just a higher level of quality in each that you may not be able to quantify.
If you want to (or can) investigate an answer for yourself, you'll just have to go look through them.
If you want to research some before doing so, study the best versions of either binos or spotters you can and apply that experience to what you're willing to afford. Though possibly dangerous for your wallet, it will help you know what you're getting out of your investment, or where you're willing to compromise.
Just like any optic, remember to try to experience the whole thing - edges, resolution, eye strain, color, ease of focus, etc. The better ones will just be easier to get behind and stay behind.