Is it because it is easier to memorize the advice: "insert the бы after Eсли and the second verb". It is perhaps more symmetrical?
In fact "бы" is placed after affected word. At least in this context.
However it is difficult to me to express slightest difference in meanings... At the top level they all express subjunctive.
I try to explain, but I may be wrong in translation:
Если бы я был писателем = IF ONLY I was a writer... ("if" is stressed)
Если я бы был писателем = If it was ME who was a writer... ("I/me" is stressed)
Если я был бы писателем = If I had a chance to BE a writer... ("be" is stressed)
Если я был писателем бы = sounds not good. but sounds as "If WRITER was my profession..." ("writer" is stressed).