The programming language of Sage is Python, which is a better overall language than the custom math-only language of Magma. Also, there are over 80K packages for Python on Pypi compared to a handful of packages for Magma. Ecosystems...

I currently use Magma in my research. I have run into a crash bug I cannot fix or debug, holding up an element of my thesis work. A lot of effort has gone into working around Magma limitations, which Sage doesn't have because it is sanely designed. If you've never had to use Magma, consider yourself lucky.

Magma is closed-source.

See Slide 4 for why it's important to be open source.

TL;DR: Researcher A finds things he wants improved in Magma (closed source) but can't. Researcher B tries to write improved FOSS implementation, but lost his job, likely in part by spending too much time writing said code and not doing other things like writing papers. Researcher A moves on and has a successful academic career. Moral: writing FOSS can cost you your academic job; it's safer to find something else to do.

Right - would you rather rely on a theorem which the author claimed is true, but won't let you see the proof, or one where the proof is published and reviewed? That's the choice between closed and open source math software.