Why didn't Rod Reiss just eat Eren? Why did he need Historia? If Reiss just ate Eren, he would be the king, and do whatever he wanted (except that the King's will may overrule some his own free-will).
In the Anime, he even says that "I must not become a titan myself". src: Attack on Titan Season 3 Episode 44 – "Wish" @ 13:46
In an interview included in the guidebook Attack on Titan ANSWERS, Hajime Isayama revealed the reason why Rod said he did not inherit the Founding Titan himself: "He thought of himself as just an observer, and in order to preserve the Reiss bloodline, he may have thought that he needed to stay human and make more children. He would have to become the Founding Titan, and he was most likely afraid that he wouldn't be able to fight against the First King's ideology, just like his father and Uri weren't able to."
It is not explained in the anime yet, it is explained later in the manga.
Chapter 88 major spoilers below
Ymir, who was royalty of the Fritz family, came into contact with the “source of the universe” and gained power to become the first Titan. The Curse of Ymir references the fact that if one of her descendants become a Titan, they only have 13 years left to live. This is because Ymir died 13 wars after gaining her Titan powers, and no one shall contain more power than her.
Rod Reiss was aware of what would happen if he became a titan, and ate Eren. Even if you are anime-only it explains that Rod was a coward, and was scared of inheriting the titan ability since the moment he looked into his brothers eyes and "realized" what it meant. He knew that once he became a titan, he would die 13 years later, and that his mind would be altered to the beliefs by the power of the 1st king. He was so afraid of these things that he was willing to sacrifice his own children to prevent himself from succumbing to these curses. The reason that he sacrificed his own children, and not random people he could've easily persuaded was because he was aware that in order to access the "true power" of the founding titan within Eren, you must be of the royal bloodline. This is why he was so intent on Historia. After she denies him they reveal that he had little to no love for Historia herself, but more so the blood in her that could've prevented him -- or at least given him an excuse temporarily for not inheriting the founding titan.
