Why did I fade in my half marathon?

I didn't even read your post, but the most probable answer is that you went out too fast.

You aren't as fit as you thought you were going into this race is the most probable reason

You wouldn't have faded in your half if you would've gotten a low taper fade prior to the race

You basically went race-pace for your long run. And you probably haven’t run many long runs to that distance.

you’re unprepared to run that fast/far… you went out too fast.

Your weekly mileage needs to be increased through a few more longer runs.

You touched on several good points.
If you want to run 6:34 pace I would be careful the first 6 miles. With your conditioning go a little slower not faster the first 6 miles. Yes you likely went out to fast.
Yes you may have needed a race to expose what you need to work on. Try pushing the last 20-25 minutes of your long run.

Yes you should shorten your long run to 7-8 easy and reduce or skip fast work for at least the last week, if it is an important race.

Fueling during the race should not be a concern in the half at your pace, but yes you do need to eat before the race.

“You went out too fast” is self-evident, but B seems like a likely tangible reason why you couldn’t maintain your goal pace.

I don't think you need to touch on half marathon pace at the end of long runs, but you certainly need to do longer long runs.

13.5 miles is only just beyond the distance of the race. I don't think you'll build the necessary durability to reach your full potential at the HM without doing at least a half dozen runs between 15-17 miles.

Also, how are you running your threshold? And how are you determining what threshold is? Some ways are more effective than others.

My guess is it wasn’t a great day for you, which meant that you started bang on what could be a good overall goal tempo that was ultimately just too fast. Because it’s a long race and a few seconds per mile too fast could mean you blow up, a small difference in your form that day can make a big difference. Not sure how you figured what your threshold is, what your HM goal pace could be, and what kind of volume of threshold work you’ve been doing, all of which can play a big role here.
While it’s tempting to look at your best workouts and calculate a goal tempo from there, it’s probably best to back off a bit from that, even just 3-5s per mile, so you can negative split and get faster in the latter half of the race. Or, if it’s a subpar day, you can at least avoid it turning into a death march.

In no particular order: Your taper was too short, your long run didn't go far enough, and you started out too fast. Everyone's different, but a HM is a long race and for an A race, I taper mileage starting 2 weeks out, with my last big workout 10-12 days out.

The HM is also the ultimate "lest the pace come to you" race. For 10K, you've got to be close to pace from the start, and in a marathon the pace that comes to you will turn around and kill you at mile 18. But in a HM, I've had a lot of success starting off the first 1-2 miles ~10 seconds off goal pace and then drifting down to where I want to be over miles 3-4.

If it's self evident, then why do so many runners still do it? Pacing properly is so rare it's like a super power.

You kind of answered your own question. You decided to just hop into a half, without a ton of specific workouts & without a normal taper, & expected to run to full potential? You should expect to be off a percent or two like that. Add in some better workouts, get the long run up a couple of miles, think about pacing/nutrition, & you'll knock some time off.

You chose to slow down...

Few stuff to ask your self

-Did you went out to hard ?

-Is 55 miles a week enough?

-Do you do tempos?

-Are your long runs supposed to be 13 miles ?

55 mpw is a hell of a lot for a 1:29 half

Drop back down to the 5k and get down to 19:00. You need to figure out what works for you. Mileage isn't everything.

What do you mean it's a lot? If I can handle that mileage with no injury why should I not train at that level?

It's a meme