Even after he made it in the UK Bowie still couldn't crack the States. His US Ziggy tour was a mostly unsuccessful attempt to have him fake it until he made it. He had to nearly go disco to become a commercial success in the US. Bowie seems to have been the beneficiary of a degree of early patience from record labels that was fairly extraordinary even at the time.
So, you can look at it as Mercury took a chance, didn't succeed and dropped him. RCA tried again and succeeded. But, he changed his approach and style around this time in an attempt to market himself better. Then, EMI would reap the rewards again in the 80s for Let's Dance which hit number 1 in a half-dozen countries and close to it in a few others.