The blood donors become more limited as research increases. When I started giving a couple years ago, there were often women donating plasma right next to me. A short time later, women were not acceptable as plasma donors due to certain antibodies that may form in their blood when pregnant. The chance of the antibodies increases with each pregnancy. Because women may become pregnant and miscarry without ever knowing they were pregnant, the plasma must be tested for the antibodies. Currently, it is too expensive to test, so they use only male plasma donors.
As an example of the expenses in collecting blood, I am a “frequent plasma donor”, which lets me donate plasma twice each week. To be in this program, I get an annual physical from their doctors. Every four months, I get an SPE protein analysis. Each time I donate, the plasma is tested for (I believe) 14 infectious diseases. The “donation kits”, which include all the needles, tubing, and containers for collecting and filtering my blood are costly as well. The platelet kits cost $250, but the plasma kits are a bit less.
Each additional test that must be performed on the blood is an added cost. The blood collectors must weigh the available donor pool versus the cost of additional tests. As @Sarcasm noted, it is a supply/demand situation. If the supply drops, the cost per unit of blood increases, and it becomes cost effective to add more tests.
Would you rather watch from the sidelines and have the safest blood supply possible, or have the honor of giving life (and enjoying free cheese, juice, and cookies!), only to find out that someone received your blood and ended up with some horrific disease of which you were unaware when you donated? Unfortunately, they simply cannot test for everything.