If space is easier than the deep ocean, how is it that the Trieste (the submersible that explored the Challenger Deep) was built in a year on a budget of millions by a single shipyard, while it took massive, multi-year, multi-billion dollar, national efforts to go to space?

Difference in difficulty of getting there. The pressure vessel for space could be far simpler than going underwater. But it is way harder to get to the point where you’d need a space pressure vessel in the first place

National pride was not at stake if Trieste failed.

There is no launch vehicle to design in the ocean.

The tyranny of the rocket equation. Or simply put you don't have to carry fuel to carry fuel to carry fuel to get you to the bottom of the ocean. Simply carrying some ballast will do just fine.

The pressure hull isn't the major engineering challenge in getting to space.

Because getting into the ocean just requires some ballast. You can jump overboard any ship at any time in a diving bell and see how far you make it. Or not. But going to orbital velocity is another matter entirely. But once you are in space vs once you are in the deep ocean the environment will try to kill you in entirely different ways and in that sense space is easier than the deep ocean. Some parts of earth are off limits with present day technology and the Titanic is roughly on the border of what you can do with some degree of reliability. And probably less reliability than was forecast. With space, once you are out there given enough fuel the solar system is your for the taking. Until you dive down in another gravity well.

Until recently at least, there had been more people on the moon than the bottom of the Mariana Trench. (There’s been a flurry of descents in the last few years, so by now the stats might have flipped.)

https://en.wikipedia.org/wiki/Mariana_Trench