Let me take a guess. They sale many more 1500 than 3000 due to the price difference. And since the 1500 is more in line with size/price that home owners are going to pay, as compared to all the other brands, then they put a big engine in it to appeal to all those who think such a huge engine is needed to mow a 1/4 to 1/5 acre yard. Just consider the game played with the auto industry when it comes to engine hp. By the way, I have an 18 hp in my 2000. It does just fine. I've cut grass/weeds easily 1 foot and higher with it.
Very nice the way it is set up. If I run too fast in such thick stuff, the blades might stall out, but the engine idle isn't affected. Now I have broken two deck belts over the past two years, but that's only about a 5 min. change out. No big deal.