I assume you mean the fact UA does not release more upgrade inventory in advance?

Here is one good reason: There are people (really) who buy paid business on this route. I know for a fact some of these people would not pay for business class if they had the certainty of confirming an upgrade right then and there. The uncertainty of an upgrade means UA gets more revenue from some of the 1Ks on the flight. Nothing wrong with a policy that helps UA earn money in my book.

As someone who wants more for less, I am willing to put up with uncertainty as part of that package. Aren't you?