As mentioned in the comments these gates have to be manned so there may not be enough officers:
At least in the UK, there’s still an officer monitoring the automated gates, perhaps one officer per every four gates. For that reason I’d guess that dynamically adjusting the number of gates according to demand wouldn’t be entirely trivial, so the number of open gates will be set to handle the average demand. Queues will then form in times of peak demand.
Other reasons could include:
Some of the gates are faulty
Not sure if this has anything to do with this but the UK has allowed several new countries to use the passport gates, see here
Security reasons, so all the gates can be closely monitored
Crowd control, so the luggage reclaim and customs aren’t crowded, also for security reasons
Credit:stackoverflow.com‘
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