Well it really stems back to the creation of the schedules way back when - maybe even as early as 1955? I can't remember when the early evening bulletin was introduced but it was certainly still in the "Non Stop" theme music era before 1976. In those days the early evening news was at 5.50pm. In 1976 it was extended by 5 minutes and began at 5.45pm. Then it was shifted to 5.40pm in 1989 to make space for the new ITV National Weather forecast. That's how it stayed until the 1999 revamp. I can't really give you an answer as to why the early evening bulletin was so early and so short, but I think to find the answer you would have to look back quite a way.
Didn't TV close for an hour at 6pm in the early days, hence ending daytime rather than starting primetime with the news made sense. As time developed the 6pm hour was predominantly local so left 7pm onwards for network content.
The Evening News now generally has a runtime of less than 20 minutes compared to 28-30 mins for News at Ten.
Up until 1957 there was something called "Toddlers Truce", where television was not permitted to air during roughly 6-7pm each night. There were exceptions, such as cricket, other sporting events, some religious programming.
BBC and ITV hated this restriction, as they wanted the audience hooked at 6pm, and stay with them for the night, so the government relented, and in 1957 the restriction was lifted, except for Sundays. 1958 saw the Sunday restriction eased, and allowed religious or "morally uplifting" programming to only air in the "God Slot" as it would be nicknamed.
Remember also until the autumn of 1972, broadcasting hours on UK television was very restricted and tightly controlled by the then Postmaster General. 50 years a go in 1968 the rule was 7 hours of normal programming Mondays to Fridays, with 7.5 hours for Saturday and Sunday each, amounting to just 50 hours a week. Sporting events had their own special annual quota of around 350 hours a year allowance, whilst schools, colleges, adult education, state occasions, political coverage, Welsh language programming were all exempt from the restrictions.
The restrictions were eased gradually in 1971 with an increase to a 8 hour daily allowance Mondays to Sundays, before being lifted all together in 1972, which allowed ITV to launch a proper, codified daytime schedule on Monday 16th October 1972.