MRAM has long (like decades) been a viable memory technology. The biggest problem with it is just that most companies that work on MRAM don't work together, so it tends to not go far. It has been used in a lot of small embedded designs, especially those where non-volatility and high tolerance for heat and radiation are important. Mainstream usage has been low despite it's many advantages. It can be made far denser than SRAM while being similarly fast, non-volatile, having lower power consumption, and not being too difficult to produce, but like most memory technologies that didn't quickly hit mainstream, it never really got enough fab allocation. The companies don't really like spending the big money on products that they aren't 100% certain they can sell, so they mostly just go on producing products they're already selling, like DRAM.