Because the trains were not showing up. If a train showed up, there was likelihood of a delay. It was also common that rush hour trains were short - so even if the train arrived, you were unlikely to be able to get in. It was a lottery with odds against you. I was constantly late because of that and employer could only be patient for so long. I am disabled (not visibly) and using Thameslink as an alternative was exhausting (as mentioned - small seats, very uncomfortable). If I recall also Thameslink had much slower service than Southern. I hate driving, so imagine how bad it was if I found eventually going by a car was much better experience for me.

So Southern wasn't turning up and Thameslink was poor?

When was it?

2017 onwards

e.g. https://www.bbc.co.uk/news/uk-england-42707560

So after it was nationalized. Southern and Thameslink are the same company.

You suffered from the political decision that the government made to get rid of guards. No company would take it as a franchise because they knew the fight with the union would impact both profits and users, so they got GTR to operate it under a management basis.

This is now what is happening to every other TOC.