Cleaner air (hence less lenticular distortion) and no city lights to spoil night time viewing.
Galileo's primary advantage was the use of a telescope. He was the first person to record astronomical observations made with the recently invented device. Aside from a few burning candles and oil lamps, there would have been almost zero light pollution back in the early 17th century. There would have been a lot of haze from fires used for cooking and heating, however, near any sufficiently large town.