What exactly prevented Goku from learning UI with Popo?

[Yuji]

You know, outside of it not being a thing yet.

The principle is the same, and the manga even references it. I don't think the problem was his power, or else Roshi wouldn't have been able to use a pseudo-version of it. Even if it was his power, he got progressively stronger in the series since then. The implication is that at some point he lost track of these principles, but I'd say at the very least until the Namek arc he was fighting with them in mind.

What exactly prevented him from achieving it before?

[Koitsukai]

Lack of experience comes to mind. Experience in controlling his emotions, not fighting experience.

It's particularly hard for a 14 year old boy to know how to control them. Even if saiyans and earthlings are different, Goku's body still was about to grow up for the "first" time, that probably didn't help either. Clearly the correct mindset isn't easy to attain, he needed the help of two angels to get there as an adult, so it's no wonder that as a child wasn't able to fully get there.

I also think he had some of it going on, some form of autododge sorta speak, just incredibly inferior to the actual technique. Like Popo did, he was good at it but still not fantastic with it.

[Desassina]

I think that Goku gained some self awareness over time, and it allowed him to fight, not for his emotions, which led him to the path of power, but for his own well being.

[Anonymous Friend]

Popo cain't teach you what Popo don't know.

[Lionel]

During the 23rd tournament preliminaries Yamcha, Tenshinhan and Krillin were acknowledging how Goku had calmed his heart and made himself almost completely unreadable. The principles of Ultra Instinct were there if not in relation to a technique that wouldn't be conceived of until decades down the road. As to its real time benefit at the time I suppose concealing one's intentions and making themselves more difficult to follow would be some of the motivations for such training along with maintaining your composure.

[Shorty GZ2]

Anime wise? He hadn't broken through the limit of his inner shell yet to unlock his deeper "fight good" potential by being hit with something so lifethreatening like his own Spirit Bomb before. Manga wise, I guess he hadn't accumulated enough martial teachings & knowledge under enough masters to reach that enlightenment or self actualisation yet. Plus the accompanying self control to an appropriate emotional shock.

[Goku9001]

In respect to the manga, it has to be something that pertains to the teachings of Angels in the same way that Beerus' teachings that acted as a catalyst for Ultra Ego during Vegeta's battle with Granolah. Vegeta's mind has always been on destruction. He had no regards for anyone, was trained to kill, and had no qualms destroying anyone who opposed him. That mindset would have been apt for obtaining Hakaishin power because there was nothing that was restraining him emotionally. However, he never obtained Ultra Ego and explicitly links that power to the training he had undergone with Beerus. I'd imagine the same pertained to Goku. All of Goku's previous masters honed his mind, spirit, and calmed his movements which are apt for Ultra Instinct. However, we are shown that one of Goku's masters is Whis when Goku recalls his teachings. I feel as though that makes it implied that Whis was a necessary catalyst for Goku's Ultra Instinct. That's also implied when Goku's training with Merus awakens his Ultra Instinct in addition to Vegeta no longer training with Whis because he wanted to pursue a path that deviated from Ultra Instinct.

Popo was a necessary part in developing the characteristics Goku needed for Ultra Instinct. But, without the proper teachings of an Angel, I don't think it would have been possible to learn Ultra Instinct. Even with Roshi's great wisdom and decades of experience, the most he could muster is a technique akin to Ultra Instinct. However, it was not Ultra Instinct.

[Saiya6Cit]

I think that Goku gained some self awareness over time, and it allowed him to fight, not for his emotions, which led him to the path of power, but for his own well being.

I agree.

Biologically speaking, the human brain does not reach it's full development until the age of 25. Saiyans are very similar to humans so it could be one of the reasons. Also, physiologically he was being affected by hormones which can really cloud the mind.