Persons vs. people

[Lyalya]

Test No. errors/inter-15 "Sleep well", question 5

By the time she had got to the shop there was a long queue of persons waiting patiently to get inside.

(a) By
(b) had
(c) persons

Test No. errors/inter-15 "Sleep well", answer 5

By the time she had got to the shop there was a long queue of people waiting patiently to get inside.

Correct entry: people
The error was: (c) persons

You have not found the error.
By the time she had got to the shop there was a long queue of persons waiting patiently to get inside.
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Why word "persons" doesn't fit in here?

[Torsten]

Because the word persons is only used in official documents such as police reports.

TOEIC listening, talks: Advertising an inflatable plane seat