Why is Tricare and ChampVA exempt from ACA coverage for dependents until they are 26, ends at 21?

The short answer is because TRICARE and CHAMPVA are covered in different sections of Federal law which give different stipulations on coverage. The governing laws have to be changed in order for the program to be changed (and in the case of TRICARE, was).

The long answer is more convoluted and comes down to legal opinions on the applicability of certain sections of the law. The Public Health Service (PHS) law was modified by portions of the ACA, one of which was the section that extended dependent coverage*.

Specific to your question, The ACA provision extending health insurance coverage to dependent children until age 26 did not extend to TRICARE beneficiaries nor to CHAMPVA beneficiaries.

There have been subsequent laws passed that expanded coverage to meet the intent of the ACA, such as the authorization of the TRICARE Young Adult Program in the 2010 NDAA (there were provisions that were put into the NDAA in subsequent years for CHAMPVA, but the never made it to the final version (blame whoever you blame when the politicians do something boneheaded).

You may ask "What was the reasoning behind the legal opinion" and it comes down to the difference between "health insurance" and a "federal health care entitlement". TRICARE and CHAMPVA are both defined as a "federal health care entitlement" programs* which have restricted eligibility based on certain criteria (for TRICARE, you have to be in the military or retired from it. For CHAMPVA, you're the family of a disabled veteran or one who died, etc.).

So ... clear as mud now?
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* TRICARE and VA Health Care: Impact of the Patient Protection and Affordable Care Act (ACA) https://sgp.fas.org/crs/misc/R41198.pdf
* 42 USC Chapter 6A - Public Health Service laws - https://www.law.cornell.edu/uscode/text/42/chapter-6A
* Section of the PHS law about dependent coverage - https://www.law.cornell.edu/uscode/text/42/300gg-14
* Unlike health insurance, a federal health care entitlement programs is financed from federal trust funds or paid out of general revenues and the federal government is legally obligated to cover them.

I wholeheartedly agree.

It seems like it's up to Congress to change the law for TRICARE - and the way the law is written it only allows up to 21, 23 if in college or longer if the dependent has special needs.

There was an attempt to get rid of premiums on TRICARE Young Adult until 26...but it doesn't seem it made it far.

https://www.military.com/daily-news/2021/06/09/key-lawmakers-want-get-rid-of-tricare-premiums-dependents-under-age-26.html
From 2021: "Momentum is growing in Congress to allow military dependents to stay on their parents' Tricare health plan until age 26 without paying monthly premiums.

Sen. Mark Kelly, D-Ariz., along with Sen. Jon Tester, D-Mont., and others, has introduced a bill that would eliminate the monthly premiums now required under the Tricare Young Adult health program."

"But under current law, dependent children of active-duty or retired personnel lose their Tricare eligibility at age 21, or age 23 if they are full-time students.

They have the option to purchase coverage under the Tricare Young Adult program up to age 26, at a cost of $459 a month for Tricare Young Adult Prime and $257 for Tricare Young Adult Select."

And prior to that in 2010 it was initially addressed after ACA passage
https://www.jacksonville.com/story/news/healthcare/2010/10/12/effort-stalls-cover-tricare-dependents-age-26/ [login to see] /

"Language was inserted into the defense authorization bill to raise the age of eligible Tricare dependents to match the new health care law. The House passed the bill, but it got blocked in the Senate by Republicans opposed to a separate measure repealing the military's "don't ask don't tell" policy for gays serving openly.

So the matter remains unresolved until Congress returns to Washington after the November elections."