All nice and easy but the true reason is that Miller hadn't planned those issues out and made it up as he went along. That's where all those contradictions come from. Also notice, the shift in tone in the last two isseus when you start to notice a lot of "Watchmen" creeping in.
Actually Frank Miller had planned those issues out and didn't just make it up as he went along with no goal in sight. He explained in Comics Interview #31 (1986) that from the beginning the final issue was the most planned and prepared ahead of time. He said he often figures out the resolution first, where he wants to get, and then figures out how to get there.
In Comics Journal #101 (August, 1985) during an interview that took place in early June, 1985, Frank Miller revealed that in Dark Knight Returns an arrest warrant would be issued against Batman, which doesn't take place until the third issue.
In Comics Journal #101 (1985) Frank Miller revealed that another aspect of Dark Knight Returns would be about Batman being "too big" to be judged, which Commissioner Yindel comes to see what Gordon had meant during the riot in the last issue.
In Comics Journal #101 (1985) Frank Miller revealed that Superman would be in Dark Knight Returns, very different from Batman.
Superman and the president at odds with Batman first appear in Dark Knight Returns as early as the second issue so that was also a precursor to things to come, further indicating that Miller did have things planned out in advance.
In Amazing Heroes #102 (1986) it's explained that ideas for both Batman: The Dark Knight Returns and Batman: Year One were based off of Miller's original 1984, early 1985 notes for his proposed revamp of Batman for his and writer Steve Gerber's proposed Metropolis line of comics.
In Comics Journal #101 (1985) Frank Miller explained futher about Batman: The Dark Knight Returns being based on some of his notes from his proposed revamp of Batman for the proposed Metropolis line.