Every platform has more than one UI library but there's two (closely related) fundamental differences:
1) there's typically just one canonically recommended for a given use case at a given time
2) they tend to a better job interoperating with one another & conforming to a consistent desktop paradigm than popular Linux equivalents (see e.g. running qt apps under gnome)
This is all largely because they come from singular entities - achieving the above would require more effort & coordination from Linux folk (though KDE do a decent attempt - Gnome has a lot to answer for here).
1) there's typically just one canonically recommended for a given use case at a given time
2) they tend to a better job interoperating with one another & conforming to a consistent desktop paradigm than popular Linux equivalents (see e.g. running qt apps under gnome)
This is all largely because they come from singular entities - achieving the above would require more effort & coordination from Linux folk (though KDE do a decent attempt - Gnome has a lot to answer for here).