> Why did they keep rebuilding it in different places?

I am pretty sure they re-routed the trail to the earliest-melting zones on that slope.

Look at the picture below created by Bob R, a composite of 4 photos taken from Wotans Throne. From 2009-06-23 Wotans Throne (picture #30 in the set)

Near the top, you can see the old trail coming up and to the left from the major snow field. That trail would not completely melt out until late in summer. Note that the switchbacks now stay pretty much in the already-melted areas.

Click here to see the full-size picture.