Because the right to bear arms is specifically stated in the Constitution. The right to life is not, and as it is not spelled out in the Constitution, the states get to decide... not Washington.
The only way you can think that is if you ignore the Statement of Purpose of the Constitution, or if you think babies are not persons.
"We the People of the United States, in Order to...secure the Blessings of Liberty to...our Posterity, do ordain and establish this Constitution for the United States of America."
"No person shall ...be deprived of life...without due process of law..." - Fifth Amendment
"No State shall...deprive any person of life...without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws." - Fourteenth Amendment