why did my battery health capacity decrease so fast?

Define so fast? If your tag line is accurate and you own iPhone Xr, it was released in 2018 and discontinued in 2021. Depending on when you bought it, even if in 2021, two years ago, one would fully expect the health of the battery to be in the 80% or lower range at this point.

Most users experience a decrease in battery health of 1-2 percent per month; this is very normal.