Officially, the change was made because it was deemed unsafe to have such a long circuit with high speeds. Back in the days, it was considered an increasingly important issue that the safety car and medical car could take a fairly long time before getting to an accident, especially if that would happen at the far end of the track.
I also think that they wanted to have some extra corners, towards where we now have the Mercedes grand stand. As it further increased the length of the track, the eventual layout ended up being a very compact circuit.
I was there once during the GP, about 10 years ago, and having visited Spa 3 times before that, I was amazed how little you had to walk to go places

And to conclude, I don't think this is a boring circuit as it is now.