I think a big reason is that people are using the rest of the amazon ecosystem. If your costs aren't primarily storage, you might be willing to pay a premium to use something that integrates nicely with other services you're using. Here's an article[0] that does some other comparisons between providers and mentions things like upload speed and security features.

[0] https://www.cloudwards.net/azure-vs-amazon-s3-vs-google-vs-b...