Octave[1] covers most of Matlab uses, and it's FOSS. And there is Sage Math[2] that integrates many different FOSS components to be something like Mathematica. Both are developed quite actively and covering many differences with proprietary analogs every year.
> I still don't understand why Wolfram Mathematica doesn't have wider adoption.
High prices for hobbyists who are not students.
The Wolfram store does not even give you the option to pay with anything else than credit card (in Germany, other methods of payment are strongly preferred), so you have to use a reseller if you don't have or don't want to use a credit card for payment.
Side story concerning the previous paragraph: I got a voucher from Wolfram Research to update my Mathematica license via the Wolfram store for a price that seemed fair to me at that time. So I wrote to the support that I would love to accept their offer/voucher, but have no credit card. Wolfram Research support told me, this is not possible; I have to use a reseller in this case (for which the voucher does not apply). OK, I get it: Wolfram Research does not want me as a customer.
[1] https://www.gnu.org/software/octave/
[2] https://www.sagemath.org/