But the main question remains: Why don't they utilize the progess we made in decades of research for better programming languages?
The argument that Solidity should be useable by the average programmer doesn't hold, in fact, typing makes programming easier since it clarifies data structures that are implicit in languages like JS.
A lot of the limitations of Solidity are due to constraints of the EVM, but the EVM is evolving. For instance, the next planned hard fork will enable the EVM to pass dynamically sized data (e.g. solidity arrays or strings) between call stacks. Previously arrays had to be fixed-size, so you'd have to define a fixed maximum size, then always return data of that size (usually a lot of empty elements). This feature, like many others, is somewhat challenging to design because every execution step of the EVM must be metered by a "gas fee"; the first version of the EVM kept it simple by only allowing return data to be a fixed size. See these issues for background https://github.com/ethereum/solidity/issues/164 https://github.com/ethereum/EIPs/pull/211
Also, the longer-term proposal is to adopt WebAssembly for EVM 2.0: https://github.com/ewasm/design. Then users can write contract code using any language with an llvm compiler (rust, ocaml, etc.).