Short answer: Because we're not bringing 50k people.
Long answer: I don't think a lot of peoole realize how expensive opening up a concourse is. That's why so many tarp off sections on their oversized stadiums. Plus you're asking them to spend this money, find the people to work, train them, and change all their logistics and planning for a singular game. Probably also against their contract with the Raiders too. If we're not going to fill it, spoiler alert: we won't, I bet they've done the math and decided it's not worth it. The 200 level makes sense since the concourse would be open anyway for the suites and club sections.
UNLV has a world class hospitality and event planning education at their school. They know what they're doing. They're not stupid.
I would love to be proven wrong and we just have a ring of cyclone fans to the rafters but assume people running football games know more than us.