Why did investigators initially assume that the crash of Air France Flight 447 must have resulted from a major equipment malfunction?

Flight 447 was passenger flight of Air France from Rio de Janeiro to Paris. It crashed into Atlantic ocean on 1st June 2009, leading to death of 228 passengers. Its wreckage was removed by Brazilian Navy and the flight recorders were recovered two years later. According to the French Investigation agency BIA's report, the malfunctioning in the autopilot was the reason for its crash. The crash was deadliest aviation accident till now.

The investigators assumed that the crash of Air France Flight 447 resulted form a major equipment malfunction because other air crafts with the same type of air speed detectors had experienced problems with the tubes freezing up in the cold, moist air.