First, it was very hard to communicate the value prop to customers. We went to market with something like "we refund the buyer's commission minus our flat fee". This brought up all kinds of questions and often buyers thought we were more expensive than realtors. Saying that we're 50% the price of a traditional realtor ended up being a more compelling message even though there's less savings in many cases.
Second, when we're acquiring customers we compete with all of the people charging a much fatter commission. If a realtor is making a 50k commission, they can afford to spend 10k on acquiring a single customer. Assuming an efficient market (which the acquisition side kind of is), a flat fee kind of caps your addressable market because you can only compete with ads at a certain price point (buyers at <400k price points in CA for instance). In some ways this is counteracted by the fact that you can save $1M buyers way more money with the flat fee, but only if you can communicate that message effectively.
That said, it looks like redfin found that there's more price elasticity on the sell side. The seller has likely been through the process before, seen how little work agents do, and feels the full agent fee more viscerally at the time of listing. For the buyer, the agent fee is this murky thing that your agent will tell you is paid for by the seller... first time buyers usually don't find out how much it is until they sign the final closing docs. The other difficulty is that the typical 2.5% being offered to the buyer's agent is essentially an advertising cost... in the existing market agents will often steer clients away from low commission listings (not ethical but it happens).