Because games with zombies sell better and are easier to make. 
Now seriously, remember how many zombie games and, zombie mods and games with zombies as iconic enemies have been released in the last 5 years? Between them there are some really popular games (from Minecraft and Half-Life 2 to CoD and DayZ). Lovecraft and eldritch horrors don't have the same kind of popularity (at least not right now).
Now seriously, remember how many zombie games and, zombie mods and games with zombies as iconic enemies have been released in the last 5 years? Between them there are some really popular games (from Minecraft and Half-Life 2 to CoD and DayZ). Lovecraft and eldritch horrors don't have the same kind of popularity (at least not right now).