It's a running joke that Wolfram is a jobs program for math PhD's. The difference isn't necessarily technical, but the sheer amount of labor that has gone into adding more edge cases and niche use cases. Sympy is great but like most open source, it's created by volunteer maintainers supported by donations.
I imagine the difference is even bigger in things like solving ODE's/PDE's.
I imagine the difference is even bigger in things like solving ODE's/PDE's.