He made lots of sacrifices to make buddhism a religion the idea started to spread because siddhartha was a very important person in his country so the idea of buddhism started to spread to also started to spread because of Trade routes for ex silk road trades it traveled through water,land etc as time went on it got faster.
To conclude after 1500 B.C buddhism and hinduism were a very big religion back In the day and they still are. For example china still follows buddhism and is still being practiced till this day and the siddhartha is one of the key reason why it is still being practiced. These two religions were founded in two different time eras no where near each other. Buddhism was founded around the 6th century (501 AD - 600 AD). Hinduism was founded In 1500