It’s all about business. The advent of films on TV is when this trend died out – when viewers could be lost to annoying opening name-checks.
Also at the same time, many of the unions and guilds for below the line staff involved in filmmaking demanded certain credits for their as their contracts became more and more complex. Thus the need for credits to be at the rear of a film grew. Imagine a film like ‘300’ with all of it’s CG artists and other technical crew listed at the front. You’d never get to see the film.
The reason you see only above the line folks at the beginning of the film is also due to contract negotiations. Even the length of time on screen, exclusion of other credits while one is on screen, position above or below the title and other factors are part of the bargaining process.