Thanks for the great question Haotian Liu. I’m sure a lot of people will be asking us this so here goes!
Here are some of the important distinctions that we see between what Piazza has been trying to do and what we're trying to achieve.
We intend for Qwerty to be treated as a resource and Piazza is usually treated as a requirement instead of a resource. What I mean by this is that many students usually attribute Piazza to something that the professor has autonomy in directing and as something they 'have to do'. A lot of what we're trying to achieve with Qwerty, by not having it go the "official University sanctioned app" route is that we want Qwerty to be a resource and community for students above all else.
Qwerty comprises perpetually existing knowledge communities curated for the benefit of the student. On the other hand, Piazza's actual implementation is contingent on the college and even then individual professors' can choose to actually implement it for their specific classes. Like you pointed out, engineering professors usually try to integrate services like Piazza in their class. However, a number of professors in non-engineering streams often never get to implementing it for their class. This implies that a LOT of students never actually experience anything like Piazza all 4 years at college. This "on-off" switch, in our opinion, creates disparities between how students learn. Also, a number of professors often choose not to implement Piazza because they too think it's more of a hassle to implement and monitor than what its worth. These are also usually the professors who don't have an army of TA's at their disposal. So even professors think about it as a requirement.
At Qwerty, we're trying to also focus on the bigger picture in terms of knowledge communities comprising students in the same major. Piazza helps with some individual classes but not really with the larger sections of people in your major. We think there's tremendous value in connecting students in the same areas of study as well.
Qwerty's UI/UX is being designed specifically to be intuitive for students to use. The functionality of Piazza's UI/UX isn't really the best. (In the 4 years that I've been in college, I've never seen an updated UI/UX for Piazza). Additionally, based on the students that we spoke to before developing Qwerty, we realized that Piazza's functionality and their UI isn't really conducive to finding the best questions and the top answers/advice/resources in response to those top questions. It's also difficult to follow interesting discussions that may be happening in these classes. So, unless you're in a specific class and directly a part of a specific interaction on Piazza its difficult to keep up with other meaningful interactions which could also be beneficial to you.
Continuity is an issue with Piazza because once you're out of a specific class, you lose access to the Piazza network for that class. With that, you potentially also lose access to the students who you thought were helpful in that class. On Qwerty, we're trying to give you this continuity by being a part of all the communities that actively interest, instead of just those classes that you're taking and your professor switched on Piazza for.
I could think of a few more reasons, but maybe I can tempt you to sign up for our early access list instead ;)