I think it's just a matter of preferences or bad past experiences.
I like Bootstrap as well but sometimes it just feels too constraint by its base layout classes. Just tried Tailwind and feels more flexible and simpler to use but I'm not saying Bootstrap is not easy to use.
Also with Tailwind you don't worry so much about what's behind the classes because a flex class will always mean it's display: flex;.
In the end, what matters most is what you feel more comfortable working with or what allows you to accomplish better your objectives.