Several reasons...
Friend A... I drifted because post having DCs I found her too needy, expected me to mother her & go out of my way to meet her, she text me constantly ahead of meet ups to guilt trip me into travelling to meet her, meaning I couldn't concentrate on the friends I was with at that time. Plus I got sick of her banging on about "her business" in a pretty hip venue of the time. She had her own business for 3 months & sold nothing. She worked for me for the other 6 years there, so people thought my business was hers. which was all the more galling as she was never great at her job, but I let things go as she was my friend. I realised that she was quite narcissistic, so I backed right off. We are back in touch now though & it seems to have reset the boundaries. She no longer expects me to mother her or bangs on about her "business"
Friend B... I cut off dead as she proved herself to be a very toxic & jealous manipulator, who tried to ruin my wedding, tried to cause trouble between DH & I & made a pass at him on my wedding day. I'd also trained her in my own time to do a job that I later employed her for. Post DD I went through a sex discrimination & constructive dismissal case with my ex employer... B was the only witness against me, by this time she was openly boasting about "bringing my smug ass down" she had a real issue with my success & happiness, so no way were we having her in our lives again... & I won my case too
Friends C, D & E
This is more recent & has been hard tbh & we'll see how it pans out long term, but for now I no longer consider them friends. They are part of a bigger very long time friendship group who have been sucked into the dark side of the covid conspiracy theory nonsense. Unbeknown to me, my sharing covid information posts from family & friends working in the frontline of covid science & care, pissed them off. It was a particular surprise with C, the one I'm closest to as I knew she was struggling with finances due to lockdowns & I helped her set up making & selling masks to supplement her income, which was successful for her. I had no clue at that point that she was a covid denier
. I was initially stumped as to why after over 30 years of friendship she left up an attack on me from a misogynist we both know. His rant was about how he doesn't know me & how I memorable I was etc because he's a sad fecker who doesn't like to be called out & deleted. He'd unblocked me just long enough to read my comments & then blocked me again... 3 other mutual friends who knew us all messaged me to let me know about the post... Friend C didn't delete it as I would certainly have done for her... I was really puzzled until the rabid covid denier antivaxxer, frequently barking mad posts kicked, not until some really schoolyard bully type antics from the 3 of them, one now clearly fancies herself as some sort of guru
... we live in weird times, so I'm keeping them at arms length for now. We no longer live close to each other & no gatherings thanks to covid. Not sure how I will feel until this is all over, trying to focus on this being a fear response, but the school yard bully stuff & some of the name calling was a bit of an eye opener, plus I look at them now & realise things with some of them now grate on my tits anyway. Hissing about hating kids, whilst babying dogs even to the extent of elaborate birthday parties is making me feel pretty "meh" about it all, which has surprised me as these are over 30 year friend ships
F came out as a full on Quanon inspired covidiot & could talk of noting else... we silenced that one, but hard to respect that level of stupidity from an otherwise intelligent man, fear response or not, so we'll see.
G is extremely CEV, I was in close contact as she was already in isolation due to cancer treatment pre covid hitting & she came out of isolation into lockdown. I was, I hope a good support to her & went out of my way to cheer her up. She was planning a holiday to visit us as soon as she could. I made her easier to wear WHO spec masks, which she loved
Then she got out into the real world, somebody got to her & she started posting covid denier, the vaccine is killing people & it's only flu type memes & suddenly blocked me on everything when I provided the science to contradict her posts... that one I feel very sad about & just hope she gets through this & I feel very angry with whoever got to her 
Covid certainly culls friendships in my experience 🥴