Why is the Panama Canal Zone not considered a Toxic Exposure Risk Activity, even with all the evidence, plus 19 Congressmen requesting it?

I'm sure it's a political decision

It was in the PACT act and they took it out. I joined the Military Veterans Advocacy .org, they are fighting to get Panama as well as other locations covered.

I assume you are referring to the push to include the Panama Canal Zone (PCZ) as a presumptive location as an exposure to toxic chemical (specifically, claims that Agent Orange was used as a herbicide there) for veterans who served there and later develop health conditions related to Agent Orange exposure.

The short answer is that "all the evidence" has been mostly anecdotal so far and/or has not been accepted as objective by the U.S. government, and the evidence that has been considered hasn't established exposure as likely under the VA's rules of evidence for creating presumptive conditions. For the VA to have a presumptive finding, there has to be clear evidence that something occurred and has been accepted by the United States government.

While there is undisputed proof that Agent Orange has transited the PCZ, the DoD hasn’t established the PCZ as a ‘test, use, or store’ site for Agent Orange*.

The other case where a presumptive exposure might be given is if the frequency of occurrence is statistically extremely unlikely.

As a hypothetical example (you can easily point out issues with my quick math … it is only used as an example), in a letter that the Congressmen you referenced sent* to the VA, they mentioned (and I have no clue if it is accurate or not), “at least 400 veterans who served in the PCZ have now developed …” issues over the 41-year period (January 1, 1958 through December 31, 1999).

During the 41 years, what is the occurrence of those illnesses developing in the non-Veteran population? According to the Center for Military History and U.S Army South records, the troop strength between 1958 and 1999 averaged out to be about 14,000 a year, so a SWAG would be somewhere around 200k different individuals served in the PCZ during that timeframe. Factoring that into the 400 veterans that have issues that comes to be about a 0.2% occurrence. Is that 0.2% occurrence significantly higher than the non-veteran population such that some other exposure is probably the reason?

This is by no means an opinion on if they SHOULD have accepted certain evidence – only an answer to your question of why the PCZ is not considered a presumptive location for toxic exposure.
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* https://castro.house.gov/imo/media/doc/letter_to_va_secretary_on_panama_canal_zone_veterans.pdf
* Armed Forces Pest Management Board – list of DoD locations where tactical herbicides were tested, used and stored outside of Vietnam - https://www.publichealth.va.gov/exposures/agentorange/locations/tests-storage/index.asp
* Criteria to be designated as a “tested, used, and stored” for Agent Orange - https://www.acq.osd.mil/eie/afpmb/docs/tactical_herbicides/Criteria.pdf
* CMH – The Panama Canal - https://www.history.army.mil/html/books/panama/panamacanal/CMH-70-115-1-PanamaCanal.pdf