Why did the WMAP mission last so much longer than Planck? NASA endorsed 9 years of data taken with the Wilkinson Microwave Anisotropy Probe (WMAP). The High Frequency Instrument aboard the Planck satellite ran out of coolant at the start of 2012, after about two and a half years of operation. Even if the Low Frequency instrument runs for another two years, its operating life will be much shorter than WMAP's.
Why is there such a big difference between the operational life-spans of the two missions, given that they have basically the same objective: to measure the cosmic microwave background?
 A: Dan Neely gave an excellent answer.  I wanted to add that WMAP was initially planned as a one-year mission, and soon extended to two years---in which it reached its designed sensitivity goals.
After that point, it continued due to its success and the reasons listed by Dan. 
A: From Wikipedia:

Passive thermal radiators cool the WMAP to ca. 90 degrees K; they are
  connected to the low-noise amplifiers. The telescope consumes 419 W of
  power. The available telescope heaters are emergency-survival heaters,
  and there is a transmitter heater, used to warm them when off. The
  WMAP spacecraft's temperature is monitored with platinum resistance
  thermometers.

Because no coolant was used, there was no hard deadline on its operational lifetime.  Its design lifetime was for two years of data collection.  However the cost of continuing to collect data from an operational satellite is low so there was no pressing need to turn it off; and until Planck had returned its first dataset, adding more WMAP data was the only option available for scientists studying the CMB that wanted more data.  However, diminishing returns come into play; each additional scan of data from the same instruments gives a smaller return.  Planck's more sensitive instruments rapidly surpassed the quality of its data; at which point there was no reason to keep running it.
Cryogenic cooling reduces thermal noise.  Using it was one way the Planck team could increase the sensitivity of their instruments.  Planck completed five complete scans of the sky before exhausting the coolant for the HFI.  The LFI is expected to have another 6-9 months of operation before its coolant is exhausted.
Finally the amount of coolant used isn't a finely tunable parameter.  Rockets come in a relatively small number of discrete sizes; you can't just spend a few tens of thousands of dollars to add a few extra kg of payload.  The next larger rocket size can easily cost millions of dollars more.  As a result, most missions are designed around the payload capacity of the rocket they're budgeted for and have a fixed mass as a result.  An extra kg of coolant means removing a kg somewhere else.
