You can argue that they should have had more Tarly scenes to hammer home his reasons for switching sides, and in a typical 10 episode season that probably would have happened, but I think it makes perfect sense why he switched sides even with there only being a single scene. ESPECIALLY in todays political climate in both the U.S. and Europe. The populous sentiment is "fukk them foreigners, I'm rolling with people that look like me even if I hate them," and that's the same appeal being used on the lords of the reach.
Cersei is on some "I know most of you hate me. But would you rather roll with me as ruler, a white noble from a prestigious house just like all of you, or the daughter of the mad king who has a slave army even though we abolished slavery because we're good moral nobles(
![troll :troll: :troll:](https://www.thecoli.com/styles/default/xenforo/smilies/troll.png)
) and 100,000 savage brown-skinned Dothraki coming into our lands. Have you heard about their culture? Savages! They even eat the hearts of their horses! They know nothing about cities and civilization, all they know is stealing cattle, destroying crops, taking your wives and daughters as rape slaves. And the mad queen? She crucified the rich cultured leaders of essos. Y'all might not like me, but at least I'm not going to change your way of life and force you to live amongst 100,000 savages for winter when there will be food shortages
![hubie :hubie: :hubie:](https://www.thecoli.com/styles/default/xenforo/smilies/hubie.png)
You decide
![ufdup :ufdup: :ufdup:](https://www.thecoli.com/styles/default/xenforo/smilies/ufdup.png)
"
Xenophobia is something that has proven, in history and today, that works. Fear mongering works. "Othering" people works. Propaganda works. Appealing to nationalism works. It was also why so many nothern houses didn't want to roll with Jon because he had wildlings with him.
Even with that, I still don't think Tarly abandoned the Tyrells in glee, he just looked at his shyt options and chose the one that seemed most reasonable given the circumstances, and that's putting the now extinct house to sleep, inheriting Highgarden for his family, and leading his country against savage foreign invaders.