Why we are saying “uh” less and 'um' more

That's my first assumption. It's less strenuous and breathy to sustain an um, at least for me. There's a clear parallel to duh (like uh) usually concerning topics of [obviousness and stupidity] and hmm (like um) usually concerning topics of [deliberation and intelligence].

Nobody knows and not even sociolinguists seem to care.

To be fair, this is what BBC News call their "Magazine" section, which tend to be even less rigorous than the already paper-thin reporting of the main news site. But it's no coincidence that Magazine stories are treated as part of BBC News overall, and the clickbaitiest headlines routinely end up in the right-hand "Most Read" and "Most Shared" panels.

Our technical writing prof in school made it his personal mission to wipe those filler pauses from our lexicon. It was remarkably difficult to adjust at first - you don't realize how much you do it until someone calls you on it every time - but I definitely found presentations and such came across much more clearly without all the uh, er, and ums. It feels very uncomfortable as a speaker to simply pause silently, at least at first; to the listener though (unless you're unusually awkward about it) it seems natural, and can single-handedly make you come across as more thoughtful and compelling.

The filled pause is actually sending you information; specifically, "this thought is not complete, but I'm putting a lot of thought into choosing the right word". It's a verbal "hourglass" icon.

I agree overuse can be distracting. But I try to think of it as flattering that they respect me enough to take the effort to carefully vocalize their thoughts rather than blurting out the first thing that occurs to them.

This doesn't always hold, but a trend I've noticed at work that someone talking up the chain of command is more likely to use filler words than when that same person is talking down the chain of command.