DNA replication is called semiconservative because an existing DNA strand is used to create a new strand.
DNA is a double stranded molecule. When DNA is copied the two strand of DNA (old strands) separate and new complementary nucleotides are added on the two separated strands.
This process creates two identical double stranded DNA molecules . Each DNA molecule contains one strand of the original DNA molecule and one newly synthesized (made) strand. Therefore, DNA replication is called semiconservative.