The answer is very simple, modern day France is much closer to Rome than modern day England, received much more Roman colonists than England, and had far greater cultural exchange with Roman society than England (and other Mediterranean societies for that matter).

When the Franks conquered Roman Gaul, it was just that, Roman Gaul. It had been occupied, colonized and influenced by Romans for many centuries. This was also true for England, but to a far lesser extent. What also accelerated France’s Romanization process was that the invading Romans killed off a large chunk of the native Gallic populations (an estimated 1/3 if I recall correctly.) Meaning there were also far less Celtic people practicing Celtic culture to counteract the spread of Roman culture in France compared to England.

Oppositely, when the Anglo-Saxons reached England, it had been Rome’s most distant frontier, separated from mainland Europe by a sea, with a fully stocked Celtic population. What was more, the Celtic peoples in modern day Scotland and Ireland were never conquered by the Romans and continued to be a safe haven for authentic Celtic culture that could counteract Roman cultures influence on England.

This explains why England is less Roman, but why is it more Germanic? There seem to be a number of reasons for this. First, there seems to have been immigration from coastal Germanic peoples to Britain before the large scale Germanic invasions happened. For example, Frisian settlements have now been found in south-eastern England predating Anglo-Saxon invasions. Other tribes probably did the same, meaning Britain was exposed to Germanic culture for a longer period of time than France was.

What’s more, the Anglo-Saxons displaced a lot of Celtic people and pushed them towards Wales, Cornwall, the Isle of Mann, and Scotland, meaning there was less cultural interchange between the Germanic invaders and local Celtic peoples compared to France, making English culture more authentically Germanic. Britain was then also invaded by Normans and Norsemen in the medieval period, who were also Germanic peoples, further reinforcing the lasting power of Germanic culture in the British isles,

Lastly, France’s established geographical and cultural proximity to Rome led to its Frankish rulers (like Charlemagne) proclaiming themselves the inheritors of Rome (and its large empire). This made a lot of sense because Gaul had been ruled by Romans for a long time, and ultimately was a contributing factor to the latinization of Gaul in the medieval period as well. The Anglo-Saxons on distant Britain on the other hand had little interest in or use for associating itself with the Romans that came before them. The fact that all of the Anglo-Saxons tribes (Anglos, Saxons, Jutes and Frisians) were not from Roman territory and interacted very little with the Romans compared to the Frankish tribes that lived on the Roman borders and eventually settled in the Roman controlled Germania Inferior before conquering Gaul probably made a large difference in influencing this too.

Short answer: because the Saxons, Jutes and Angles basically replaced the entire Brythonic speaking peoples in England, while the Franks didn't replace the original inhabitants of Gaul (who spoke a Proto-romance language) but were in fact outnumbered by them. Basically the Anglo-Saxons didn't share their new territory so they could stick to their own language, whereas the Franks took over most of the local language. The Brythonic languages were pushed to the outskirts (Welsh, Cornish, probably Pictish but I'm less familiar with Scottish history). Reminders of Frankish are visible in place names, for instance, like obviously the name of the country. Most of the Frankish language is actually visible in Dutch (as a direct descendant of Old Low Franconian).

Sources: a history of the French language, Peter Rickard, and Fennell’s A history of English.