To me this question assumes more intention than exists. There isn't a document somewhere that spells out why all the defaults exist. They're the way they are because that is the way they were left the last time someone looked at them. If there was an inciting event behind any of the changes, then it has been forgotten. That is assuming there even was a change, and not just a value inherited from BA 15 years ago.
The main principal behind this sort of thing is that units shouldn't look broken at Level 0 (
https://zero-k.info/Forum/Post/226780#226780 ). This includes units seemingly not doing what they are told, and doing 'unreasonable' things on their own. This is augmented by observing what people struggle with, especially new players. I can't just use my own judgement because I prefer to have everything set to hold position by default.
It's particularly hard to give an answer because the question seems to be wrong. I just tried it, and the only artillery that defaults to hold position, and that can fire while moving, is Merlin, Envoy and Lance. Here are some ad hoc reasons that this makes sense compared to, say, Sling:
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As a strider, we expect people to have few Merlins and to pay more attention to them. It is also quite reasonable to want a Merlin to fire from less than maximum range, to counteract its spread.
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Lance is prone to missing and teamkill if it fires while moving, it also takes a long time to turn around.
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Envoy makes the least sense, it was probably just forgotten when whatever change went through.
Git log would know.
