If you want an honest answer, it's because #ALM is not philosophically grounded. It is, in essence, a reactionary movement only in response to BLM. This makes it seem not genuine. In other words, there is nothing grounding All Lives Matter if Black Lives Matter were to disappear tomorrow. If BLM is gone, ALM would disappear. Weird! (This isn't necessarily true, but is my intuition).
Notice here how this is very different from the capitalism vs socialism fight. Neither is necessarily reactionary. That is, both have solid philosophical grounds on which to stand. Both can be advocated for without necessarily invoking the other.
Imagine if there as an organization that is calling for the proper care of treatment for Army Rangers after a combat injury--Ranger Lives Matter. Then some Air Force guy takes offense to this and says well "All Military Matter"!!!!!!! Of course, Rangers aren't saying only Rangers matter, that would be dumb and a straw-man. We are just calling for the proper treatment of Rangers after combat injuries. That doesn't mean other people shouldn't start organizations to help drone operators with their PTSD. It would, however, be weird to adopt the slogan Drone Operator Lives Matter but only after Ranger Lives Matter exists. On my intuition this seemingly reactionary slogan makes it seem less genuine and in good faith.
I'm not here to play team red or blue. I don't care. I'm just trying to explain how #ALM as a slogan has bad optics for a civil servant without invoking "you can't care about all lives until you care about black lives". Hopefully, I have done that here.
