The reason basically boils down to the fact that revamping an existing package to meet the needs we saw appeared to be a greater investment of effort than just writing this from scratch.
We hope that the design decisions we made will make future people less likely to do that and more interested in building on top of what we have. I think the fact that ModelingToolKit.jl now uses SymbolicUtils.jl for it’s simplification pipeline is an encouraging sign that we might be on the right track.
I should also add that we would likely not have been able to write this package if we didn’t learn from existing previous attempts at similar things. This is a hard problem and we shouldn’t expect anyone to ‘get it right’ on the first try. We’re optimistic about this attempt, but only time will tell how it turns out.