Between 2001 and 2004, 105 countries provided troops for peace operations (including non-UN operations, conducted by regional organizations such as the African Union or on the basis of special agreements, such as the Multi-National Force in the Sinai). But 21 of the countries provided an annual average of fewer than 100 troops, and while small detachments are of value in some missions, most mission require a lot more troops. Only 75 countries contributed an annual average of 100 or more a year.
Many countries just don't have the troops. There are 158 countries with ground forces of 800 or more troops. For a country with 800 troops, even keeping an annual average of 100 on peacekeeping missions is quite a strain. And even when a country has a more substantial military establishment, that doesn't mean it's necessarily much of a help in peacekeeping.
Poor, underdeveloped countries, unstable countries, countries with authoritarian regimes, are all much less likely to contribute peacekeepers than reasonably developed, stable, and, particularly, democratic nations. Even some reasonably prosperous, stable countries, don't have troops of the quality appropriate to peacekeeping.
So the burden tends to fall on a relatively small proportion of the world's countries. A handful of countries manage to average about a thousand troops a year. This includes many in Europe, the US, and Canada. In Asia, India, Pakistan, Bangladesh, South Korea, Malaysia, and Jordan do well, but otherwise the record is poor. In Oceania, Australia, New Zeeland, and Fiji are important, but most of the rest of the countries are too small to contribute, as is the case with the Caribbean states.
Despite often large national armed forces, Latin America is hardly represented, nor is Africa. Latin America has just never been all that interested, although the few Latin American peacekeeper forces that have served, and done quite well. What Latin American and African do participate in is non-UN, regional peacekeeping missions. But getting people to go outside their own neighborhood has proved difficult.
Although there are something like 75,000 peacekeepers on active duty today, already an all-time high, the demand is rising. But finding countries with troops suitable to peacekeeping is not easy.