You are allowed to tailor your ‘why leave’ essay to suit the particular things you like about each school. You are a math person- look at the math track at WUSTL v your prospective schools- are there divergences that are meaningful to you? Don’t gin up flimsy differences, but there may be aspects of math as a major that you get now, that you might not have as a HS student. Similarly, check out the diversity- of all kinds!! - at your prospective schools and for those where that is a particular strength and value, it is fair to say that you underestimated the importance of that to you.