Please give a simple list of reasons that extraterrestrial intelligence may not exist or is very rare

Conditions that evolved intelligent life on planet Earth may be exceedingly rare in the Universe. These reasons for that are speculated to be:
1. The solar system is in a safe location within its galaxy.
2. The location of the earth relative to the sun and other planets in the solar system gives it protection from extreme environmental conditions that would prevent life from occurring.
3. The steps that led to advanced life forms (prokaryotic cells to eukaryotic cells, sexual reproduction and the Cambrian explosion) may be very unlikely to occur.