It is extremely difficult to utilize a bicycle when a pedal or pedals are missing or inoperable. A missing chain means the rider must find alternative means of to move the bike forward. Most solutions to this problem are far less efficient and will draw attention to the biker. A damaged chain can make the bike totally inoperable by jamming the drive train and if, instead, it constantly falls off, well then the bicycle is in nearly the same state as the problem pedal bike.
Now then. Pedals and chains. Some of the most frequently troubled parts of a bike and also the most quickly worn to the point of repair or replacement. If a nearby discount department store carries the correct type and size of these parts AND they happen to be discounted down in price, well then, one can walk away from the store with these parts for around $10 a piece. Of course, based on the theme of this q&a, one could walk away from the store with the parts without paying for them. However, with security, cameras, and more than a few people milling around, shoplifting can be far more risky than a theft as described in the question.
The thief is motivated by a desire--here a need for the parts or a need for something from someone who has a direct need/desire for the parts. Willingness to commit the theft is enhanced by opportunity. The opportunity is bandied about weighing the benefits (fullfilling the desire in a timely manner with satisfactory gain) versus the risk of getting caught (police involvement; shame: being the object of a parent, spouse or friend's ire; fear of reprisal from the victim). When the benefits out weigh the risk by a certain amount AND the opportunity presents itself, the conscious decision to act comes about, and the plan is executed.
Now the risk-benefit-opportunity triad is dynamic and interdependent. (I lump desire in with benefit as these aspects are directly proportional: raise desire and the perception of benefit increases as well. Diminish desire and the benefit appears diminished as well). The opportunity of your locked bike fell into the thief's desire- benefit wheelhouse and the risk score seemed acceptable.
Pedals and chains: high maintenance, low on the fun/flash scale, high on the necessary equipment.
Easily stolen to satisfy a need for another bike .