Because if you want to change a countries base language then you need to bring a whole bunch of working class people, the vast majority of the population, with you when you do it. Just bringing in a mass of mainly top tier individuals and their retinues proved in this case insufficient. Outside the castle gates almost everyone continues to speak the local language. Eventually numbers tell.

There may have been other structures working alongside this, although its hard to be sure what effect they may have had.

At the time there was no formal learning structure (schools), most people couldnt read and write and the bureaucracy was mainly limited to the administration area. Thus there are no schools where you can demand all children learn French from 5-18, no street signs renamed, no govt. paperwork that ONLY now comes out in French, no literature/papers etc. Thus the 'peasants' had no incentive to learn French or even an easy way to learn it. The (very small) middle classes may have had an incentive in an attempt to ingratiate themselves to the Norman overlors, but obviously not enough to trickle down more than a number of words that integrated into English rather than replacing it.

We know HEAPS less about the AngloSaxon period, with nothing much more than a few words here and there and nothing pertaining to this issue. Thus there is a lot of argument about the details, most of which is indirect (DNA, Linguistic studies) and thus subject to interpretation, how many Angles, Saxon and Jutes, what position they held, why did their language win, was the West German grammar altered by exposure to Brythonic etc. But putting it on a base level it appears that AngloSaxon 'won' because it was introduced at a lower level, and perhaps in greater numbers than the Normans (who largely went around marrying one another and keeping things inhouse to a degree) and at some point it was easier for everyone to get on well together by speaking the same. Its hard to quantify but I suspect the AngloSaxon ruling classes (as much as they were) were also much closer to the people than in Norman times, thus providing a bit more incentive to be able to speak to the boss. The old belief that the Angles, Saxons and Jutes just 'pushed' the Britons into Wales and replaced them is no longer credible. It was an admixture, but we/they are still debating how much and why.

Note. This is a simplification of a very complex situation.

Because it's not a superstrate language.

The linguistic model of language transition is that substrate languages adopt superstrate (prestige) languages rather than the other way round - this means that the new language doesn't adopt many loanwords, but the old language adopts many new words. Eventually, the old language dies out, which is what we see in France and in Scotland, where Gaulish and Pictish is replaced by Latin and Gaelic. When the Romans invade Britain, much of the Celtic language disappears and Latin is adopted for whatever reason.

That's the model and you can see it in effect above: if Old English is a prestige language, then it will become adopted widely, and have very few loanwords from Latin and Brittonic. When the Anglo-Saxons arrive in England, virtually no Britonnic or Latin is absorbed into the language because Anglo-Saxon is a prestige language for whatever reason, and Latin/Brittonic are substrate languages. What's even stranger is that Anglo-Saxon place names at least, move into areas where there are not many Anglo-Saxons - Wales and Cornwall have a high level of AS names but they're on the fringes of where the AS mostly settled - but that's what you'd expect with a prestige language. Anglo-Saxon speaking is also suggestively linked with being a prestige-class - Britons are worth less than an Anglo-Saxon, so there will be some effort to socially improve yourself, which will inevitably require language acquisition.

It's also clear from later writers (Bede), that we are all Anglo-Saxons really - so there's a sense of identity about speaking Old English, a point reinforced by Alfred's reforms later on, and the centralising of the English state (!) during Alfred's time and beyond probably plays a part in reinforcing a sense of identity. There's no similar thing in Norman England with French, and it's difficult to see any love for the French who have suddenly replaced the entire top level of English society, with the attending replacement of all things Anglo-Saxon- church dedications and local saints are all replaced by the French which indicates that the local culture was beneath them - again, not a way to win friends and influence populations. Only about 20,000 French come in (1% of the population) and while numbers for AS England are unknown, I suspect that the lack of organization of the tribes probably plays a part in the adoption of a migrant force.

Old English survives in Norman England for a while, but French becomes an elite language and Latin is used for governmental writings, so England is trilingual until about the 1150s, and Old English mutates into Middle English, adopting a large number of French and Latin words, but retaining its position as the primary language for the population.