Or maybe the democrat party has delivered more on its agenda than the Republicans.
I don't think that it is debatable that Democrats have done just that.
But part of the reason why is that they generally agree on what that agenda is and ought to be, and in their ideological uniformity are better able to leverage their communicative and organizational advantages with a news media that itself largely shares their values.
Another part of the reason is that many elected Republicans at the national level must live and work in Washington, DC, which is basically Mordor-on-the-Potomac, controlled at all levels (except for Congress, sometimes) by Democrats.