Why are the minimums between these two ILS instrument approaches to the same runway so different?
The equipment requirement (in the aircraft) are different for each of these approaches.  In the upper-left box of the Z approach, you can see that DME is required to make this approach.  This requirement is not listed in the Y approach. 

If your aircraft has the right equipment, you can reduce your minimums by selecting the appropriate approach plate. 

In this particular case, the missed approach procedure from the lower minimums is more precise. The procedure includes DME distances that must be used while executing the missed approach procedure.  Without DME onboard the aircraft, you would have to use the less-precise missed approach procedure requirements of intersecting VOR radials. This lack of DME, and thus a less-precise procedure, results in higher minimums. 

In Section 4, para. 140 of the TERPS, the reason for multiple approaches to the same runway, using the same navigation system (ILS in this case) allows better-equipped aircraft to gain an "operational advantage".