I'm someone who always tries to get behind the logic of every grammar point, and for Japanese, my endeavor has been fruitful most of the time. But not always, for example when わかる is used as an imperative, as in わかってくれ, for example. What is understood is marked by が in the case of わかる, so わかる doesn't have the meaning of actively "understanding" something, but instead of something "trickling down" and being "dissolved" and thus unterstood. At least that's how I've always understood it.
Here's my question: If it's not about actively understanding, how does it make sense to say「わかってくれ」to another person? After all, they are not the agent of わかる. Instead, the thing understood is the agent.
My guess: わかる is one of those words (like 好き) which theoretically demand が, but where the semantics of the words themselves have made them quite susceptible to grammatical change in that regard, making it far easier for them to tolerate を. わかる originally just meant "to be divided" as far as I know, so of course it would take が. But its current meaning might be disconnected far enough from that original meaning for it to now mostly be perceived as just "to understand", where the usage of が that comes along with it would only be an old grammatical relict that is starting to vanish in casual speech. I presume English might have had some influence here, too. All of that is just an educated guess, but it seems plausible judging from my experience.
Thanks for reading! This could have been a two-liner, but I always end up writing half an essay...