Why did "which" lose its L (it was "hwilch")?

Much of English is Germanic in origin. German retains the "l" in "welche" (which).

I suspect that there is a tendency in English to lose sounds that aren't needed - especially in common words. You can put this down to laziness. Of course there are many words that retain their old spelling but are pronounced differently today. https://www.grammarly.com/blog/words-that-are-not-pronounced-how-they-are-spelled/

Note that, during the 20th century, the word "which" lost its "hw" sound in many places. Now in England I hear only the pronunciation "wich" although in parts of Scotland the "hw" is preserved. Maybe one day "wich" will become the normal spelling.

We can see this in "could" where the "l" is silent. There is no clear reason for "which" to lose the "l" and for "could" to retain it, albeit silently. English spelling is something of a lottery.

People have been trying to regularise English spelling for over a hundred years. I suspect this will happen naturally owing to the proliferation of phonetic spellings on social media.