Why is tofu so popular compared to tempeh?
One possible reason is that Tempeh is uniquely from Indonesia, which is much less of a culinary influence on the US than China, Japan, and Korea - all of which use tofu in traditional dishes. There were an estimated 95,000 Indonesian immigrants in 2010, where Eastern Asian immigrants was over 3.9 million in 2014.

sources:


https://en.wikipedia.org/wiki/Indonesian_Americans
https://www.migrationpolicy.org/article/asian-immigrants-united-states
https://en.wikipedia.org/wiki/Tempeh