Roman Spain, France and Italy all were conquered by Germanic tribes (Visigoths, Franks and Lombards respectively) who created kingdoms there in late antiquity. These kingdoms all ended up speaking the language of the general population and abandoning the Germanic languages of the ruling class.
A similar invasion occurred in post-roman Britain but in this case the kingdoms kept their Germanic languages and it was the native Romano-Celtic languages that were abandoned.
Is there any explanation for this difference?
Britain was abandoned by the Romans. Around 410, the last legion stationed there was recalled to defend the continent, and Roman civilization in the island collapsed subsequently in the face of the invasions.
The Visigoths, the Franks and the Ostrogoths all settled in Spain, France and former Yugoslavia/later Italy in connivence with the Romans, as autonomous allies/vassals called Foederati. These peoples adopted aspects of Roman culture as their own, including the Latin language. As the Western Roman Empire finally decomposed, the Foederati just gradually gained more autonomy until they carved all territory into their own kingdoms. In this case, there was no clear rupture with the Roman past, and the "Barbarians" preserved more Roman culture than they eliminated.
I don't really know much about the Lombards, but they arrived in Italy centuries after the others.