The premise of this question is questionable. The Mauritian passport is not especially strong.
The Wikipedia article quoted in the question is citing the Henley and Partners Visa Restrictions Index 2017 (PDF). While this is a reputable source, it uses consecutive numbers for rankings, even if many countries tied at previous ranking levels. It does indeed place Mauritius at rank 33 with 131 visa-free or visa-on-arrival countries. However, contrary to what that ranking might suggest, the number of countries ranked ahead of Mauritius is not 32, but rather 60. For comparison, at ranking 1, Germany has 176 visa-free or visa-on-arrival countries.
The countries ranked ahead of Mauritius include countries with very strong passports like the USA, Canada, nearly all of the EU, Australia, Japan, Singapore, etc. as well as several countries with more medium-strength passports, such as Mexico, Barbados, Uruguay, Argentina, Brunei, etc. Venezuela ranks immediately behind Mauritius.
But the question is why should powerful countries like Germany, USA etc. give this privilege to Mauritius?
The USA does not. Mauritians are not eligible for the U.S. Visa Waiver Program and, according to the wiki linked above, require a visa to enter the USA. Additionally, Mauritius is not one of the nominated or 'road-map' countries for the U.S. Visa Waiver Program.
As for Germany, Germany is part of the Schengen Area, which is a free-movement area that includes most of continental Europe. Thus, visa-free or visa-on-arrival entry to any country within the zone effectively grants it for the whole zone, including Germany. Mauritius was a possession of a European power almost continually from 1638 to 1968, including of the Netherlands, France, and the U.K. Having been part of a European country that long and that recently, its social and economic ties to Europe are strong, so having visa-free or visa-on-arrival access is not especially surprising in that case.
Mauritius does not have the bargaining power of European Union, it is not a superpower, it does not play an important role in global politics.
True, but, while those factors may help, they aren't necessary for a country to have visa-free or visa-on-arrival entry to another country. In general, tourism and business travel benefits the destination country, regardless of whether the country of origin is large, small, or a superpower. For most open societies, these forms of temporary immigration are encouraged. What destination countries do not want tends to be the following:
People trying to use tourist or business visitor entry permission to circumvent the country's immigration rules. For example, this would include people who intend to stay in the destination country for an extended period or work there. Almost all countries require a visa to immigrate for such purposes and they don't want people trying to circumvent that process.
People who can't fund their stay. Whether their intentions are to overstay or not, countries don't want visitors who don't actually have the funds they'll need for their stay and, thus, will be likely to try to draw on social programs of the destination country instead of paying for their own needs. And they certainly don't want people who might resort to crime (theft, etc.) to pay for their needs. Not everyone has a reasonable understanding of what it really costs to visit a certain area, especially in the case of people from low cost-of-living areas visiting high cost-of-living areas, so even some well-intentioned visitors may fail on this point.
People who may be a security concern for the destination country. For example, countries obviously don't want members of foreign terrorist organizations, drug runners, or hostile foreign spies entering their country.
Thus, destination countries usually consider what is likely to be ratio of legitimate tourists and business travelers using a visa waiver or visa-on-arrival program to people from the above three categories misusing it when considering whether to grant access to these programs to nationals of a given country. Even if a country is small and not necessarily a large player in global politics, if it's stable, friendly, and has citizens who can generally support themselves and will want to return home after a visit, it's beneficial to a destination country to make it easier for people to visit from that country. It's cheaper for the destination country to process a visa-free or visa-on-arrival entry and it also raises tourism and international trade income, due to people being more likely to visit when it's easier to do so. It's generally a win-win as long as people aren't misusing it.
Wikipedia also has a decent article on the concerns that prompt countries to require visas. In the cases where those concerns are not as large, countries will normally be more willing to grant visa-free or visa-on-arrival entries.
