Flexibility is the key. When NAT was launched in the 60's it was regarded as something revolutionary. Now with the phalanx of news outlets it's lost. a lot of it prestige yet public affection still keeps it in the slot:
If they made the bulletin completely flexible so no one knew when on earth it's on... 10.30pm one night, midnight the next night... that would be ratings suicide for them. It's pretty much a given that it will be on at 10pm or 10.30pm on the odd night so people know when to expect to view it. Consistency is the key.
They tried replacing the 10pm slot and look what happened. Hence why I can't believe this topic has come up on here again. The 10.30pm is only going to work when a big hitter leads into it (e.g. I'm a Celeb). You have a dud lead into the news and people will just tune into the earlier news on the BBC.
Also, 10.30pm-10.45pm really is when Mr and Mrs Average is likely to be headed for bed should they have work the next day. You have the bulletin any later than that and that's shaved off most the audience again. Again ITN tried this at 11pm and it failed.
It's not just 'legacy' why the news is on at 10pm, it's just common sense.
January 9th 1989 - December 3rd 2006