Ultimately all I can think of as to why (a question burning at me as well and I feel bloody stupid that I haven't figured it out yet):
There is some "trick" or "minor exploit" out there, that when used increases return on mining in some kind of reliable way (a statistical way to predict luck reliably on a pool using a certain share payout method for example) and because said tricks/exploits are ultimately a zero sum game in the long term, if everyone knew about the trick, then it would ultimately result in less return. But if select few know the trick, they stand to gain a lot. Those select few are the ones using GPUMAX, and they are likely making a killing. And they are unwilling to share their trick, because that means less advantage for them. Eventually the trick will get out there, and it will become the norm. When that happens they will loose their massive advantage. The trick may rely on the ability to ramp up and down lots of mining quickly, making GPUMAX still viable long-term for the people who choose to leverage such a method.
A good example case would be day trading on the exchanges. You can make a killing on bitcoin if you have a solid day trading strategy, and automated systems to allow you to trade high volume, high frequency effectively. But if EVERYONE did it, the market would stabilize much more. The volatility would be reduced, and the returns on the day trading would become much much smaller. So the few that have it "figured out" aren't sharing the "how" because if they did they would loose their huge advantage (right now an individual that can make 3% gain per day consistently is looking at doubling their money every 24 days on average. But if everyone started doing it the volatility might drop far enough to reduce that to only 0.5% per day consistently. It's still a viable means of making money, so it will continue to make sense even after the "secret" is out. And not EVERYONE will do it because it still requires a certain amount of know-how and resources. But they will keep their secret as long as they can to ensure that 3% lasts as long as possible.
I think the same thing is going on with GPUMAX. There is a "Secret" that the "Buyers" are using. They are currently making say 180% gains on it, so they are happy to pay 150% on mining shares. But once the secret is out, it might mean only 120% gains, so the average price to miners will fall to 110% on shares. It will still be viable, but for now they are being tight lipped because it secures their advantage. And the few miners who are "sellers" and may be aware of the trick (if they aren't using it themselves lol) are staying tight lipped too, because when the minority are the buyers and they can make 180% each, that keeps the overall average PPS price higher, making it more profitable for the "sellers")
I know this doesn't exactly explain the actual WHY people are buying shares. But after doing much digging around, and thinking about it a fair bit, this makes the most sense as to the answer as to why people are being so tight lipped about it.
Now if I could just figure out what the "it" is
lol so I could be in on the party... But until then I'm happy making the bonus payout on my shares mined against GPUMAX for now