Why hasn't anyone tried to court Randyll Tarly?

[devilish]

There is no way he could ever defect to Stannis. He opposed him in Roberts Rebellion. And then supported Renly knowing he was the younger brother with a false claim. He made his bed and has to lay in it as far as Stannis goes. Even if he bent to Stannis, there is no way he would ever have any position of high authority in title, lands, and overall wealth. He would still be looked at as a past traitor and cant forget that. Roberts Rebellion he could make a case for, that he was fighting for the true king, but declaring for Renly, Stannis could maybe forgive (like he has with others) but never forget, and Tarly knows it

He was following his liege lord orders. Surely even Stannis can acknowledge that because he obeyed his brother blindly despite not agreeing with him in most cases. Ultimately Stannis had forgiven all those who declared for Renly in the first place. Surely he would have forgiven Randyll especially considering he had the potential to break the Reach forces into two

[Maid So Fair]

Same thing can be said about the Freys (the twins) and the Boltons (the Dreadfort)

Both were no2 house in their region, a position Tarly is not even close to. And even then it took Robb more or less losing the war for them to defect and at least in the case of the Freys acting in bad faith with regards to their alliance first.

[Lord Lannister]

 

He didn't really rescue her. The faith handed her over and he swore a vow to return her when her trial comes

Then why did he bring his army to the city again?

[Light a wight tonight]

Same thing can be said about the Freys (the twins) and the Boltons (the Dreadfort)

Not entirely. Robb's cause is seen as a sinking ship where the Tyrells have arguably the biggest army in the richest area in Westeros. As I said, if the situation deteriorates he might desert, but it would be to advance himself. I'm pretty sure he's tired of playing second fiddle to a dimwit like Mace.

OTOH, he might support a Hightower takeover of the Reach if the Tyrells slip. He'd have to be guaranteed a bigger holding than Horn Hill, though. Much bigger, to justify the risk involved. 

[Abdallah]

House Tarly is that loyal house who always follows their liege lord. They're the Manderlys, the Marbrands of the Reach. 

[Lord Wraith]

Lotd Randyll's holdings are in the Reach. It would be political suicide for him to declare for another liege and against the Tyrells.. I don't see him as necessarily loyal to them, though. He may stage a coup to usurp the Tyrells, or maybe even the Iron Throne, if the situation really gets messed up, with an invasion by Dany added to Aegons's and Stannis coming back from the North.

This basically. Look what happened to the Florents.

What about him repairing Maidenpool? He is pretty much the only commander in the series who was trying to fix some of the carnage that had taken place in the Riverlands.

Yes because his son just married the heir to Maidenpool.  Granted it was a good thing but its probably a mix of that and to better defend it from outlaws.

Then why did he bring his army to the city again?

The sparrow movement I would think. I'd certainly want an army if I was going to King's Landing in the current political climate.

[Bloodmane]

Try looking at a map. Which castle is closest to Highgarden?

Randyl is a good general, he also has a fancy sword, but his military power is a small fraction of that of his liege lord, more important he has no one to hold Horn Hill while he is on a campaign, It would be foolish for him to rebel against the Tyrels, and Randyl did nothing to suggest that he is a fool.

[Bironic]

Why should he leave the Tyrells? Until now they were on the winning side. This might change with  Euron's attack, Faegon's landing and Littlefinger in the Vale, so we might see him switching sides before the Tyrells go down.