I looked into this about a decade about via the company Nuvasive, a maker of minimally invasive spinal fusion products / system. So I am a little dated and may get a term wrong, but this really stood out to me. https://t.co/7fmK4GzU28
Prior to Nuvasive, the only ways to fuse lumbar vertebra was either anterior (through the stomach) or posterior (through the back).
This was a messy, complex procedure that involved a general surgeon and an orthopedic surgeon, and not even all orthopedic surgeons were trained for this surgery. So, spinal fusions were much less common.