Why can't I get the expression for this karnaugh map using minterms?
There is firstly a mistake, it is not \$ab'c'\$ but \$ab'c\$. And you also have to keep in mind that the table rolls in the edges (both left/right AND bottom/top), so that the two lower 1's can be grouped with the two upper 1's. The last term \$ab'c\$ can then be further reduced to \$ac\$. Finally you forgot one 1 alone, which is \$a'b'c'd\$.