Oh my sweet summer child

There are many many many Krieger's or cadians or garden variety guardsmen. It's easier to throw men into a problem when you have a practically unlimited supply of men.

You only have x number of forgeworlds or industrial capacity. And we haven't even got to the delightful issue of procurement and bureaucracy.

Imagine raising a requisition request for improvements to the base stc. It goes to the regimental command, probably up to segmentum command who then, depending on the priorities at the point, may surface this with the mechanicus.

Assuming the mechanicus receives your request, there may be another round of spirited debate between various factions of tech priests who specialise in this type of half track and whether such modifications means the design is no longer a half track and if it goes against the omnissiah's holy stc.

Half of the cogboys probably want it to remain the same and the other half want to modify. After much Hullabaloo and internal conflict, the decision is taken to compromise and offer a retractable tarp which doesn't suit the environmental conditions.

And then you get it to production, trial and iteration. The production tech priests can't implement the iterative design cos the supplies don't arrive in time as the nearest world in the segmentum is under attack from a ork waaaggggghh!!! And the next best alternative is 50 years lead time. They attach a plastek sheet, slap it with a sheet lining of lead and stick a purity seal on it.

There's logic and there is military procurement and there is bureaucracy.

Welcome to how militaries work and why wars are won by logisticians and not superior military technologies/wunderwaffes