The question is a bit broad and non-specific, so you'll probably have a few follow-ups here, but here goes.
France did not really "force" their colonial populations to speak French, as the continued use of local languages generally went unpunished, unlike in the motherland, where languages like Breton and Occitan were more heavily cracked down upon - but those parts of the colonial empire that had access to colonially provided education would of course learn French as their first (and often only) foreign language. However, most of France's colonial subjects at that time were impoverished and logistically inhibited and at many points, France (like other European colonial powers) was controlling territory and local warlords rather than controlling population groups directly.
Okay, so I will have to interpret the question there, because you provided absolutely no context and no example of what you're referring to, which of course makes it a bit difficult. You probably refer to former French colonies and colonial populations that still speak French as a language in their respective countries.
The following countries speak French:
Europe: France, Belgium, Switzerland
Americas: Canada, Haiti, French overseas territories
Africa, official language: Mali, Senegal, Guinea, Ivory Coast, Burkina Faso, Niger, Benin, Togo, Chad, Cameroon, Central Africa, Equatorial Guinea, Djibouti, Gabon, Congo R, Congo DR, Burundi, Rwanda Madagascar, Comoros, Seychelles
[Note: In some of these countries, French takes a secondary position, i.e. Equatorial Guinea, where the main language is Spanish. Most of these countries though operate exclusively in French.]
Africa, second language: Mauritania, Morocco, Algeria, Tunisia
Middle East, second language: Syria, Lebanon
Minority language status is granted in places like the United States, where Louisiana and Maine specifically are two states that still have a strong connection to the French language.
So, if you observe the above list, you can of course see the "natural spread" of French throughout western Europe (natural here meaning 'by the sword and by persecuting minority languages'), in Belgium and Switzerland as well as in France itself.
Canada, or more specifically Quebec, is a special case, where the country is mostly anglophone but Quebec was so full of French colonists and French-speakers that the Anglo-Canadians were unable and for the most part unwilling to prevent the bilinguality of their colony and then later on country.
But the other countries on the list are French colonies that didn't see significant French colonial settlements (with the exception of Algeria, perhaps) and that still chose French as their first language. Why? Because it was the easiest pick in the demographic environment.
You will see how Syria, Lebanon as well as the Maghreb States refused to choose French as the first language and instead gave it secondary status. That is because these places already had an incredibly powerful pick for language of exchange: Arabic. In Tunisia, Lebanon, Syria and Mauritania it is the exclusive first language, whereas Algeria and Morocco also brought the Berber language on board, taking parallel standing next to Arabic as the first language of choice.
That wasn't as easy for the countries further south however, as they were not as thoroughly islamized as long or as effectively and thus Arabic had not been able to 'naturally subjugate' the local languages and to form a supra-regional language of exchange. Not everyone in these countries spoke Arabic, and many of the people further south weren't Muslim.
And, unlike Europe, Africa hadn't seen nearly as much of a process of centralization of languages. Military conquerors had been less effective there, proving unable to reform the continent into one of great empires like Europe had been for centuries. National identities can be shaped by a common language and against a number of foreign adversaries (just like France did, for example), and Africa didn't have that. Sure, you had Kanem-Bornu, Mali, Ghana, Benin, Ethiopia, the Kongo and other local supraregional kingdoms flare up from time to time, but there was no effective uniting process that was comparable to the European one.
Next up, Africans for the most part didn't have the written word, making it very hard for dialects to form clusters around one elevated dialect. The German language elevated a dialect spoken in a region in Thuringia because that was the dialect Martin Luther used in his bible translation. Illiteracy in Africa prevented such a natural consolidation process of language - and nationhood, for that matter, as the African concept of nation at that point would have been far more tribal and thus regional than the comparable European one. Europe had fought its own diversity down, consolidating languages and tribes and small fiefdoms into bigger, more compact and more powerful empires. Africa, as mentioned, did not have that process nearly as often and in places where it did, primarily around the coast, those local warlords even helped the European conquest of Africa by selling them slaves, providing them with caravans, allowing them to build outposts like the Portuguese feitorias and so on.
Point is: French was a natural choice in these sub-saharan countries, as they didn't have an African language that could have rivalled them. The fact we can prove this is that there is a prevelant counter-example: Swahili, a language that through means of conquest and consolidation not unlike the European ones could spread as far as it did and is today the largest African language by number of speakers. It is official in much of East Africa and might even be the basis of a possible East African Federation - so that's exciting.
But sadly, the French colonies we are discussing here did not have that. By the way, I am aware that the Democratic Republic of Congo as well as Rwanda and Burundi were Belgian colonies instead of French ones, but much of the same logic applied when these countries gained their independence from French-speaking Belgium as when their immediately French brother states broke free.
Case example: Take this linguistic map for Ivory Coast. Yeah. That's simplified by the way. So, if you're the Ivorian leadership, which of these languages do you elevate above the others and use in your courts, on your road signs and in your government documents? That's correct, none of them. The South Africa approach of having 11 official languages is theoretically good for national harmony, but South Africa too has clearly chosen to not elevate Zulu over Xhosa or Sotho over Tswana, and effectively English is the language of exchange between its various ethnic groups.
And if you compare these former French colonies to those in Indochina, you will see that Vietnam, Laos and Cambodia abstained from giving French official status. That is partially because of ideological/political reasons, but these three countries, like the ones north of the Sahara, already had their own powerfully consolidated languages. Vietnamese simply was better at spreading and taking over most of the Viet people, and funnily enough achieving minority status in the Czech Republic, than many comparable African languages. There's Laotian and Khmer for Laos and Cambodia as well, where the story is very similar. Well, minus the part about the Czech Republic, I guess.
