Combination of lots of different things.
I hate to say it, but her voice... Aagh. I tended to switch her off when she came on the radio.
As a pp said, it went right through me too. To make matters worse, when she was challenged by interviewers she would talk faster and higher and louder - it made her sound panicky and out-of-her-depth. She should've done a Thatcher and had elocution lessons.
In terms of content, she was muddled. She just doesn't project authority. Tbh if she was my line manager at work I wouldn't take her that seriously.
The really superficial stuff - hair, clothes etc. Sorry, yeah, she often looked a bit of a mess. It shouldn't matter, but in politics, it makes a difference.
On a personal level I find many individual Tories deeply objectionable, but their election campaign was crystal clear (not that I think they will get Brexit done any time soon) and they appealed to everyone who is sick of Brexit and not that interested in politics, which is a lot of people. By ruling out any coalition with Labour, JS succeeded in splitting the Remain vote.
Half the population has an IQ below 100. If you want to do well in a democracy, it doesn't do to overcomplicate things.