Why did she have to be sinless? Because Jesus had to be born without the taint of original sin.
Why did He have to be without sin? To be the perfect atoning sacrifice.
Why was an atoning sacrifice needed? To atone for those subject to original sin.
Who is subject to original sin? All descendents of Adam and Eve.
So...we are all, as descendents of Adam and Eve, subject to the corruption of original sin, born in slavery to sin and as sinners thus worthy of eternal damnation. My question is why did God send His only begotten Son to die in atonement for the sins of the elect (defined in this case as all those who are ultimately saved) if He simply could have suspended the taint of original sin in all His elect from the start and eliminated the need to die for all of them? Furthermore (and I think I know the Catholic answer for this but I'll ask anyway), why not just suspend it in Christ Himself instead of in Mary?