Because they care about maintaining their voters far more than enticing non-voters. If you listen to legislators and their staff for example, the way they perceive it is that non-voters may as well not exist in their minds, but eroding voters get attention.
The bitter fact is that a winning candidate has no incentive to reform the voting system that put them in power.
Why would a dominant party want to give any competitor an advantage?
Because they care about maintaining their voters far more than enticing non-voters. If you listen to legislators and their staff for example, the way they perceive it is that non-voters may as well not exist in their minds, but eroding voters get attention.