Taking an utter guess, probably under the assumption that the fuel temp will be more or less the same in all tanks so only one temp probe is required, and they went with tank 1.
Imagine all the information you would need to display to show all the individual tank temps, which at the end of the day isn't really high on the priority list to monitor I would have thought. (well, taking into account its a 737 and not airbourne long enough for long haul fuel iceing considerations).
Just a thought.