Padmé Amidala was Queen of Naboo in The Phantom Menace. In Attack of the Clones, we see her as a Senator, with another woman as the Queen of Naboo.
Is it explained somehow in the movies or another canon source, why Padmé went from being the Queen of Naboo to merely the Senator from Naboo?
While Padme is the queen of Naboo when we meet her, she is an elected queen.
From The Phantom Menace
PALPATINE : To state our allegations, I present Queen Amidala, the recently elected ruler of Naboo, to speak on our behalf.
She served out her two terms as the queen.
From Attack of the Clones
PADMÉ: Popular rule is not democracy, Annie. It gives the people what they want, not what they need. And, truthfully, I was relieved when my two terms were up. So were my parents. They worried about me during the blockade and couldn't wait for it all to be over. Actually, I was hoping to have a family by now... My sisters have the most amazing, wonderful kids... but when the Queen asked me to serve as Senator, I couldn't refuse her.
Whether there are a maximum number of terms or not is slightly unclear, but the point is that she served a set number of years as queen and then a new queen was elected, at which point, she became a senator when the new queen asked her to.
Ohhhh, well yup! It's mentioned in the conversation between Anakin and Padme on Naboo. In Attack of the Clones, Padme was the target of a few assassination attempts in, and on the way to, Coruscant. So to protect her, the Jedi Order sent her back to Naboo with Anakin as a bodyguard, while Obi-Wan searched for the assassin.
When the pair arrived on Naboo, they went to visit the Queen to explain the situation. On the way, there is a scene where Padme and Anakin are walking together, with R2D2. And Padme did mention that she felt she had been too young when the people of Naboo asked her to serve as Queen, and she felt that she had not been ready at the time. But after her 2 terms were over, the new Queen asked her to stay on and continue to serve Naboo as a senator in the Senate. And she just couldn't refuse.
So, yup. Padme was the Queen when Anakin met her. Then she served out her 2 terms as Queen, probably while Anakin was off training as Obi-Wan's apprentice (while he was away and didn't see her again for 10 years). After her 2 terms were up, she simply continued to help the next Queen, by serving in the Senate.
Also, in the movies, the Queen of Naboo that Anakin and Padme met in the second movie, and the Queen that walked in Padme's funeral procession in the third movie, are also different. The Queen in the second movie is obviously older, while the Queen in the third movie looked a lot younger (though we only caught a glimpse of her). So the second movie's Queen had probably also served out her 2 terms, and had given up the seat after that to the new Queen. Looks like she didn't serve as Senator after that, though...
This is addressed more clearly in the (canon) novel Star Wars: Queen's Shadow. In short, Padmé had completed her two two-year terms of office and was no longer eligible for re-election to the position of Queen.
Away from the lake house, Naboo was voting. The gears of democracy were well oiled, and centuries of tradition made the biennial event run smoothly, even with the inclusion of Gungan voters for only the second time in the planet’s history. Though few of them chose to vote, Padmé knew her efforts to include them were appreciated because Boss Nass had told her as much. Loudly. Naboo was not quite as united as she might have liked it to be at the end of her four years of service to it, but the people were happy with what she had done.
“You weren’t even tempted?” Sabé had asked when the messenger had come with the amendment for Padmé to read and she had returned it unsigned after the barest of glances. It was the closest they had yet come to discussion of the future. “Of course I was tempted,” Padmé had replied. She settled back in her seat, and Sabé resumed brushing her hair. “I thought of at least ten more things I could do with another term while I was reading the proposal. But that’s not how our legacies work. Not here. We serve and we allow others to serve.”
Despite having a variety of other offers on the table and a pet project to free the slaves of Naboo, Padmé was offered the position of Senator by the new Queen, ostensibly keen to secure someone of her political calibre for the role but also (and fairly obviously) keen to cash in on Padmé's personal popularity with the people of Naboo.
“Amidala,” the queen said, “I was hoping to ask you to take the position, and to represent Naboo and the surrounding worlds of the Chommel sector in the Senate.”
Padmé was not often at a loss for words. Even when she was, she excelled at saying something perfect and inane to cover her silence. But this time, she said nothing. Her guard was down—or at least it was as down as it ever was—and it was as though she didn’t know where to look.
Though not necessarily a strict-canon answer, the question at its core is flawed. While being the Queen of a planet is a tremendous honor, she went on to become a Senator of the entire Galaxy.
The question posed here is more like asking "This person was Governor of California. Why did they go on to become just a Senator in Washington, DC?"
I put this here instead of making it as a comment as there is still the in-universe answer of the Galactic Senate being more prestigious than being Queen of a single planet.
I have noticed that she did get asked to be senator by the rising queen. I wanted to address the fact that queen seems like more of a ceremonial power, more for meeting with royalty and making alliances than rules. The senator has more of the ability to make a law or send soldiers to war than the queen.
