I wonder why they took so long to disembark the passegers, esp as the No1 engine was still running.
Maybe in part
because the #1 was running.
However, in general, using emergency chutes (even if only on one side), is not a decision undertaken lightly on a big aircraft - there will be injuries.
I'm pretty sure that the situation will have been fully evaluated (whilst retaining emergency options). The crew obviously came to the conclusion that there was no immediate danger that required the passengers to be thrown down the slides, and that an orderly disembarkation was the best option, and that the passengers would be more comfortable in seats than in 30degC humidity outside with vehicles driving around.
Deciding on what to do, then the mechanism and then getting the wherewithall (steps/marshalls/vehicles etc) all takes time. Validating the numbers is probably a bit of a red herring, but complete evacuation is a requirement, so using 1 exit is an additional crosscheck.