Or they used a radioisotope that has a much longer half life than the other manufacturers used. As I understand a lot of tubes use a bit of Krypton-85 gas as a primer, which has a half life of about 11 years. A tube made in 1960 is only 2% as radioactive today as it was when it was made. They didn't always use Kr-85 though and from what I've read some of the solid radioactive primers put into tubes bordered on being random radioactive waste.
Gas regulators can go bad even if they aren't used, both due to atmospheric leakage and due to depletion of any radioactive primers. For all intents and purposes, your Haltron tube has reached end of life while the other 3 are still good.
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Tritium is also often used, it has a half-life of 12.32 years, and paint with uranium dioxide, half-life hundreds of thousands of years or more. Still, the ratio between the Haltron start-up time and that of the others is in the thousands, not just a factor of 50.
I use the 85A2 in a series regulator, at the recommended 6 mA, fed by a resistor from the regulated output voltage, and RC filtered to one input of a differential amp. I have had no issues with startup. Output voltage is extremely stable, there is essentially ZERO temperature drift.