DEN has bad weather and flow control today. They likely could not make it to DEN
with adequate reserves for holding since DEN
has been having weather delays. MSP
is a good diversion point since Icelandair already serves MSP
so they have contracts in place for fuel and staff, and also it is very close to the typical routing of KEF
is close to the range of the 757. With weather conditions requiring more fuel, it is logical that a fuel stop is required even though it is not needed for typical operations.
If you have never designed an airplane part before, let the real designers do the work!