The other major factor is that CRTs back then weren't as high quality, especially at the edges, as later CRTs. The black borders weren't just there for size considerations (they could have used a physically smaller CRT,) they were also there because the edges of CRTs tend to have worse picture quality than the centers. This is true even on the latest CRTs, but they are made to be "good enough" at the edges, while earlier CRTs they were perfectly happy saying "leave a big black border". (You don't notice as much on TVs simply because TVs usually don't have a need to display sharply defined lines near the edges all that often.)