Does anyone know why

I'm trying to find out why it's 2 hours after eating that has become THE time o measure blood glucose. It's not to find the top of a spike, because that usually occurs much sooner. It's not to see if bg has gone back to "baseline" because the Diabetes UK information says "up to 8mmol/l" for T2. That's higher than the 4 - 7 recommended for fasting and before meals.
It must be something historic, but I can't find what. Yet I still read people writing about their 2 hour pp reading. What use is it? I can't myself see any use to this number.

I think that it's when the insulin is cooking away so, if the 2 hour reading is bonkers, you can infer that an adjustment is warranted? It's sort of when the "food peak" and "insulin peak" would be synchronized to provide a useful amount of data? It's probably less useful if you are oriented towards eating "spikeless" food...

It may also just be that "clinically", it's easier to tell people to check @ 2 hours?

I have no idea, but that is an excellent question. I do know that I've been told by at least one endo and read somewhere that if you suspect your bolus ratio is not good, to test at 1, 2, and 3 hours after finishing a meal to get a better idea of what's going on. I do know that when I was using a CGM, my "spike" was definitely closer to the 1-hour mark and by the 2-hour mark I was generally coming back down (assuming a meal relatively low in carbs).

I'm guessing that it comes from the Oral Glucose Tolerance Test where the BG at 2 hours post ingestion will be indicative of impaired glucose tolerance. Anything above 140 mg/dl at two hours is considered impaired glucose tolerance. Greater than 200 mg/dl 2 hours post ingestion indicates diabetes, which is also why I'm guessing they tell you to shoot for under 200 mg/dl at two hours post meal

As for why, it appears to be historical from everything I've been able to find out. Agreed though, I'm interested in my peak BG at whatever time post meal that may be.

Regular insulin peaked at about 2 1/2 hours and Humalog or novolog peaks at around 1.5 hours so 2 hrs was a good time to look at your BG and see if you have too much or not enough Active insulin (IOB) to cover BG rise. Remember the medical community takes a cookie cutter approach and your insulin action times could be diffrent.

I would consider correcting in the next hour depending on it's upward or downward trend. I never worry about stacking Novolog after 3-4 hours and I have done many corrections at the two hour mark without having a low BG event ...again YMMV

That's a lot more scientific answer then I was going to give! My impression is that it is close to the time that most people's blood sugar seems to hit its spike based on some complex interaction of the insulin action and the food digestion. But I have heard people on here say that their own personal spike is closer to 1.5 hours and so that's when they test. I would think it would be easy enough to keep some records and find out your own personal spike time and then make that your testing time. If you had a pump and set your insulin duration time it would then give you the correct IOB for corrections if you needed to after your pp.