People complain about best-of-1 a lot, and we hear that. We also see that people play best-of-1 a lot. More than the corresponding best-of-3 formats, every time. Even among our highly invested players, who have all the game modes turned on and spend money and participate in multiple formats, the data shows the same trend.
Our goal with the Arena Open is to make meaningful tournament play available to the greater Arena population - not just players who are already deeply invested in competitive Magic and familiar with all of its trappings. In order to reach that audience, we wanted it to be more approachable. By making Day 1 best-of-1, players can (initially) play the event the same way they play Arena every day.
Once a player has qualified to Day 2 and established that they have something serious to gain, we're more comfortable asking them to commit additional time, energy, and resources to playing best-of-3. Because we agree with the premise behind this post - that best-of-3 is a more rigorous competitive format. It's the gold standard for high-level competition, and it's important for players to still prove themselves in that venue.
All of that said, we are looking at ways to offer multiple Day 1 paths in the future (best-of-1/best-of-3 being an obvious pairing). There are a handful of issues that will need to be addressed (balanced time commitments, competitively fair structures, some tech stuff), but we're working on it.