As so many have said before, October originally WAS the 8th month. Month 1 in the old Roman calendar was originally March. Through the course of history and many changes we are left with what we have now.
Immediately after the Julian reform, the twelve months of the Roman calendar were named Ianuarius, Februarius, Martius, Aprilis, Maius, Iunius, Quintilis, Sextilis, September, October, November, and December, just as they were before the reform.
The first month of the year continued to be Ianuarius, as it had been since 153 BC. The Romans later renamed months after Julius Caesar and Augustus, renaming Quintilis (originally, "the Fifth month", with March as month 1) as Iulius (July) in 44 BC and Sextilis ("Sixth month") as Augustus (August) in 8 BC. Quintilis was renamed to honour Caesar because it was the month of his birth.
Other months were renamed by other emperors, but none of the later changes survived their deaths. Caligula renamed September ("Seventh month") as Germanicus; Nero renamed Aprilis (April) as Neroneus, Maius (May) as Claudius and Iunius (June) as Germanicus; and Domitian renamed September as Germanicus and October ("Eighth month") as Domitianus. At other times, September was also renamed as Antoninus and Tacitus, and November ("Ninth month") was renamed Faustina and Romanus.
Commodus was unique in renaming all twelve months after his own adopted names (January to December): Amazonius, Invictus, Felix, Pius, Lucius, Aelius, Aurelius, Commodus, Augustus, Herculeus, Romanus, and Exsuperatorius.
So apart from the sanitation, the medicine, education, wine, public order, irrigation, roads, the fresh-water system and public health, what have the Romans ever done for us eh?