[sedansesame]

The popular reason is likely because 文 is used across East Asia in Chinese and Japanese (Korean and Vietnamese too, though written differently)[0], with the ideograph standing in as a sufficiently different contrast to the Latin alphabet, and as a reference to a major non-latin-alphabet based user market, while being simple enough to render (compared to something more "difficult"[1] like 语/語)

It's also been used in the Google Translate logo as well.

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[0] https://en.wiktionary.org/wiki/%E6%96%87

[1] Not only because of Simplified/Traditional/Japanese renderings (文 is mostly the same across all three), but more strokes for a small icon is a bad idea regardless