Why did ancient civilizations only know about six planets?

Answer:

The ancients only knew about (5 planets + Earth) because to really see the other two you needed a telescope.

The planets they knew were:

1. Mercury
2. Venus
3. (Earth)
4. Mars
5. Jupiter
6. Saturn

All these are easily visible to the naked eye and of cause they can be seen to move against the background of the stars (except the Earth because we are on that)

It was in 1718 (290 years ago) that Uranus the 7th planet was discovered by William Herschel using a telescope. Although it is visible to the naked eye like the other six, it was never recognized as a planet by ancient observers due to its dimness and slow orbit. The 8th planet Neptune was found by mathematical prediction rather than regular observation. Unexpected changes in the orbit of Uranus led astronomers to deduce the gravitational perturbation of an unknown planet was the cause and they predicted where to look to find it.