First of all, I don't think setting a minimum threshold would be at all fair due to the disparity on how many credits each project tends to give out: GPU projects will always have an easier time clearing a fixed threshold than CPU-only projects, for one.

Even if a mechanism to adjust the threshold could be devised using the data on available platforms within a project, the inherent differences between the tasks could still result in a radically different credit output given the same hardware on different projects. For example, Bulldozer CPU's are known to be have only half as many floating point units as integer units. Meanwhile, Ryzen CPU's appear to have reversed this trend with rather strong floating point performance and unusually good encryption capabilities.

Whether or not projects clear a threshold given a similar userbase to other widely-supported projects could easily come down to the type of computations they do.

After all that, the project admin could still manipulate their credit reward computation to clear the threshold by scaling everything by some multiplier. This is one of the reasons that all projects get an equal share of the total magnitude in the first place, to make such manipulation pointless.