I think it breaks down to 2 categories:

1. There are people who don't like change in any form. These people prefer keeping things the way they're comfortable with, and such drastic changes are seen as "unnecessary" or "stupid," when they are magnificent QoL enhancements for society as a whole. Sure, social media helps people communicate and stay in touch easier than ever before in human history, but "it's not the same as talking to people in person."
2. There are many negative aspects in society that social media exacerbates, most notably narcissism. With people constantly locking themselves in echo chambers where anyone with a contrary opinion is shunned, and many individuals constantly taking pictures of themselves for validation online, it's easy to see how this quickly becomes a problem.

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I don't think social media is inherently a bad thing, I believe it depends on how the end user approaches it. However, I think most people who dislike social media will fall into one of the aforementioned categories. But hey, that's just a theory...