Why did some conquerors change the region's language and others didn't?

Question

In history we see many examples where a conquered people ceased to speak their native language and began speaking the conqueror's language, and also many examples where conquering groups ceased to speak their original language and began speaking the native language. For a long time I've wondered what factors cause either of these options to occur when a territory is conquered, and did not yet find a satisfactory answer. Here are some examples for both cases, to provide grounds for discussion:

• When the Anglo-Saxons conquered England, the native population switched to speaking (old) English, rather than vulgar Latin (or Celtic languages).
• Native peoples all over Latin America switched to Spanish (or Portuguese) after the Spanish conquest. Note that this includes the Inca empire, which was solid and well-established at the time.
• Egyptians stopped speaking Coptic and switched to Arabic after the Arab conquest.
• Gauls in Iberia, France and England switched to speaking Latin after the Roman conquest.

However,

• The English continued to speak English after the Norman conquest, and didn't switch to French; the Norman ruling elite eventually switched to English (of course this brought a lot of French influence into English, but nevertheless I'm puzzled by the fact that the language remained English).
• Many cultures along the eastern Mediterranean continued to speak Greek despite the Romans conquering them, which was an important factor in the Roman empire eventually splitting to east and west. (Compare to most of the western Mediterranean switching to Latin, as mentioned above)
• Iberia didn't switch to Arabic after the Arab conquest, but continued speaking Latin. Neither did Persia/Iran, which continues to speak Persian.
• France continued speaking Latin after the Frankish conquest; the Frankish ruling elite eventually abandoned their original Germanic language and switched to Latin. The same happened to the Lombards in Italy, where Latin continued to be the dominant language.

I'll be happy to hear explanations for these examples and other similar ones. Why were the Anglo-Saxons able to impose their language, for example, while the Franks and Lombards couldn't? What differentiates between Egypt, Iberia and Persia in terms of the adoption of Arabic? In general what factors decide or influence whether or not people adopt their conqueror's language, compared to the reverse situation?

Answer 1

Here are some factors I think could explain many cases, but as noted in the comments, the actual historic cases might be better answered by asking in History SE.

Number of Conquerors

A simple factor is just the raw numbers of the conquerors. The most simple way to learn a language is always to learn from the parents.

Take the English conquest of the North American continent for example. The influx of settlers combined with superior agricultural methods made the European and African population outnumber the native North American population. Now, most of the US is not speaking any language native to the Americas.

Contrast this with the Lombards, which could not have been so many, as Wikipedia writes on Kingdom_of_the_Lombards:

Any genetic heritage of the Lombards was rapidly diluted in the Italian population mostly because they rarely intermingled with the local population.

According to https://www.bbc.com/news/science-environment-35344663, Studies suggest that on average 25%-40% of the ancestry of modern Britons is attributable to the Anglo-Saxons, which is pretty much. And since it wasn't mentioned, I think the Norman percentage can't be too high.

This seems to be the biggest factor to me.

Ethnic mixing

If the conquerors' and the conquered population mix, the children will often learn the more prestigious language – the language of the conqueror. This then gives a boost to the most important factor: numbers. Given that almost the entire population of Latin America has some kind of Spanish descent, this is a reason to why they speak Spanish.

Connection to Homeland

Either the conquerors impose their language on the conquered, or they themselves learn their language. If the conquerors have a connection to their homeland, they have a reason to continue to speak their own language. This connection could also lead to a constant influx of new speakers.

The Arab world or the colonial powers were huge empires, but e.g. the Lombards seemingly did not have many ties to their old homeland.

Abstand (Similarity)

The closer the languages are, the easier they are to learn for the conquered. But aside from dialects, I think in pre-modern times (without mass education) only affects how long it takes for one language to take over, not which language takes over.

Ausbau

Some languages have a long written tradition, others don't. Especially the languages with holy scriptures have some bonus, as religion serves as another reason to learn the language. There was for example not one unified German language and many texts in the Medieval Ages were written instead in Latin, which had a long written history.

Lingua Franca

When the conquered do not speak one single language, the conquerors' language can then serve as a Lingua Franca. This might explain the situation in many colonies, where the colonial powers are now the official language. This often coincides with Ausbau, as the many languages of the conquered are often not ausbau languages.

If a language is used as lingua franca, but not really spread enough for people to learn it, often a pidgin arises instead.

Ideology

Do the conquerors want to spread their language? In this very nationalistic world, you can strengthen your claim to land much more if the populace speaks your language. This caused states like the USSR to settle many Russians in its vassal states and be vigorous in spreading Russian as language.

Mass education and technology

If you can afford compulsory education for your populace and can broadcast your language over radio and television, you can spread your language better.

What exactly the reasons for each case is, might be better answered by a historian.