Why are the electron affinities of the group 4a elements more negative than those of the group 5a elements?

The electron subshells in the atoms of group 5a elements discourage electron addition.

This is because the subshells in the group 5a elements are already half-filled, which is a more stable electronic configuration that that which would be achieved if another electron was added.
Meanwhile, group 4a elements' subshell becomes half-filled when an electron is added, making them somewhat stable. This means that they prefer electron addition.