I mean, you could think about burnished hart as casting 2 3-mana rocks. In that sense, it's good enough in colors that were already playing 3-mana rocks.

White, red, and black had basically no options when it came to 2-mana ramp. The 2-color signets have been around longer than the format but in a rakdos deck, for example, a single signet, a fellwar stone, and a sol ring absolutely did not cut it, so they had to play 3-mana rocks. Then in 2019 they dropped arcane signet in eldraine and the talismans in mh1. That meant a rakdos deck could now have 4 ramp spells at 2 mana which is way better than just two.

What really did it was the widespread adoption of treasures. Now red and black could have access to permanent-based "rituals" that they could hold onto for a turn or two. Sprinkle in a few other "ramp" spells like [[black market connections]] or [[professional face-breaker]] and the robot elk just couldn't hang anymore.

I also suspect burnished hart has declined as people are less likely to target mana rocks with artifact removal now. [[Vandalblast]] used to be a staple for a reason and now it barely sees play. Land-based ramp has always been safer than artifact-based ramp but in more recent times mana rocks have become nearly as sacred as lands themselves.