Why does the cdc recommend that an additional primary shot should be the same as the first two doses?

Do you mean the booster shot after 6 months from the second shot? Booster shot was advised because the neutralizing antibody (anti-spike protein antibody) drops significantly after 6 months and we cannot tell how much protection from memory B and T cells from the first 2 shots. There's certain protection from these cells which is only known if the infection occurs but it is risky to be infected.

Because it shows the biggest increase in antibody response. Good Luck