Because ABM was and is considered by most nations a threat to world peace as it increases the chance of a first strike by the party who has it. There was an Anti-ABM treaty which he US signed and then unilaterally withdrew from. US missile defense is progressing to the point of making the Russian (and Chinese) strategic arensals ineffective once it reaches that point the US can execute a first strike without fearing retaliation. Russia can't afford ABM so they are building new delivery systems, China doesn't have the tech nor does it seem to want to develop its nuclear arsenal any further. Both of them look at the US missile shield as a clear threat to their national security and regional and world peace because it would give the US the ability to do what every they want with impunity.