This is interesting of you to say, as I've had my fair share of experiences on Southeastern trains where we've been at a halt with no information. Were you using SE metro or mainline services?
It might be a 'Southern thing' but indirectly.
One of the takeaways from the Lewisham incident was to keep passengers informed during disruption. In a built up urban area, especially where a train is stopped and passengers can see buses that follow the exact alignment of the railway passing by, it's a pretty strong motivation to decamp and continue your journey via alternative means if there's nobody reassuring you that you'll be on the move shortly. It was also noted that long-distance trains that didn't have many attractive alternative arrangements (ie beyond Orpington) mainly stayed put.
Of course, if you're on a train in the North where the railway is surrounded by farmland, or where alternative arrangements are harder to come by, you're much more likely to stay put on the train no matter how uncomfortable it is. And when your train has toilets !
I'm pretty sure the Elizabeth Line drivers are told to let passengers know why they're stopped if stopped for more than 90 seconds.
I'd like to think that the first thing on the minds of managers where there incidents nowadays (espec in the Kent region) that have left trains in undesirable places is acting quikcly to avoid self-evac. But this doesn't seem to be the case necessarily (Stratford and Hither Green/Grove park evacuations) last month saw passengers on trains for longer than desirable amounts that could've (and did in Stratford*'s case) led to self-evac's.
