Why indium blue and thallium green lamps, don't have heat reflective coatings at the ends of the arctube?

Those salts are easier to evaporate, so are not that sensitive for the cold spot there.
Even on white lamp you may notice when it is operated below the rated arctube temperature, it shifts towards colder CCT. So the blue and green components are available in full, but the yellow and orange (Na,...) not yet, resulting in the yellow and red components missing.

And generally a single component lamp is not that demanding for the temperature to be that exact, as there is no problem with the component ratio being altered, it just generally shifts the efficacy few percents, which is not noticeable at all.

It also helps improve the colour saturation.  In the same way that many HID lamp types become whiter when driven at higher vapour pressures, the same is true for the coloured types.  This is easy to see when dimming or overloading such lamps.  Cooler operation results more saturated colours.  Therefore it is beneficial to use larger volume arc tubes than normal, sometimes to fill the outerjacket with gas instead of vacuum to provide additional cooling, and of course also to omit the heat reflective coatings.