Why are there such large differences in conversion rates from different sources?

I recently launched an AI Headshot product. https://www.indiehackers.com/post/how-i-made-1-023-within-10-days-of-product-launch-2ea9f56bb6 There's a stark contrast in conversion rates from different platforms, all dedicated to AI, ranging from as low as 0.5% to as high as 21%. This represents a forty-fold difference. How can this be and how might I identify more similar sources with such high conversion rates?