There are 8 billion people in the world. That means that about 0.15% of the population follows him on Instagram. And approximately 99.85% of the population does not. The average person has never heard of him.
Why does Ryan Garcia have a lot of Instagram followers? He's successful, good-looking, and he has Mexican heritage. Boxing is huge in Mexico.
UFC is big business, but it's still ultimately a niche sport, and their business strategy is promoting the UFC brand rather than individual fighters.
The boxing business model is different, with a bunch of rival promoters each trying to create breakout stars.
TLDR; The UFC doesn't give a shit about creating stars. The brand is what matters.
Boxing needs to make stars to generate interest. No one tunes in to watch a random boxing match. Lots of people watch UFC without particularly caring who is fighting.