I guess they didn't because they didn't know for sure how much would the softer and hard tires last. Starting with the soft ones provided good grip at the race start and they just needed to see how was the times progression on their car and on the rivals' to decide when to swap. It's surprising Hamilton managed to make the soft tires last more than any other car in the first stint. Perez assumed the risk to make them last a couple of turns more than Hamilton did.
OTOH they say Alonso's Ferrari got damaged in the rear part, mechanically and aerodynamically after the incident with Vettel, when he got pushed outside the track. Maybe that explains that Perez managed to catch and overtake him so easily.
Hamilton is hungry and looks much more mature, he's a very serious rival. We'll have a lot of excitement for the next races