To slightly reword an earlier comment of mine It is notoriously difficult to rigorously explain the economic development of any given country. For any simple relationship you can think of, you can find a country that has those features, and yet isn't developed, or vice-versa.

That said, Switzerland has some features that show up on general studies of rich countries versus poor: market economy, reasonable protection of property rights, relatively effective and uncorrupt government, stable money supply, and peace. 

Beyond that we can say a bit in particular about Switzerland's economic development.

For a start, relative Swiss economic development appears to have started in the second half of the 19th century, which is when railway technology was around, and railways can substitute for a lack of water transport. Switzerland is also part of Europe's blue banana - yes that is the original name - of urbanisation. economic development and industry, that stretches from London across Amsterdam, Brussels, Frankfurt and down to Milan in Italy. The blue banana includes Zurich in Switzerland. The blue banana shows that Swiss cities had access to trading partners in northern France, southern Germany and northern Italy - the rich bits. (But to what extent are these rich because of access to the Swiss?).

Switzerland's mountainous regions also make for good access to hydro-power stations: expensive to build but cheap to run. David and Mach (2006) report that in 1914 Switzerland was the second largest producer of electricity per capita after Norway.

The Swiss political system is conductive to flexible stability (yes I know this sounds like an oxymoron). In other words, Swiss institutions could adapt to new situations without civil society tearing itself apart. This, along with Swiss neutrality, helped Switzerland develop its international finance sector: for example the Swiss franc was very stable during WWI and WWII, whereas Germany had hyperinflation in the 1920s and the USA and UK dropped their gold standards.

Finally, the lack of many natural resources, apart from hydro power, in a small economy with a well-functioning political system does tend to focus policy-makers minds on supporting domestic development so as to raise taxes - other examples  are Japan (post-WWII after it gave up its imperial ambitions), South Korea or Singapore. There's a known phenomen in economics of "the resource curse", where countries with lots of natural resources often fail to develop economically, and that is attributed to, in part, natural resources reducing the need of policy-makers to worry about domestic production and export performance. 

In summary, Switzerland has many of the attributes common to developed countries, and indeed a particularly stable political system. Swiss economic development also started after railways started being a serious alternative to water transport. Switzerland also is positioned within a band of strong regional economies. Finally, a lack of natural resources, apart from hydro, probably helped concentrate policy-makers minds on economic competitiveness. 

Sources:

Beatrice Weder and Rolf Weder, Research Paper No. 2009/25 Switzerland’s Rise to a Wealthy Nation: Competition and Contestability as Key Success Factors,

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.395.4149&rep=rep1&type=pdf

Thomas David and Andre Mach, Research Paper No. 2006/101, Institutions and Economic Growth: The Successful Experience of Switzerland, (1870-1950) 

https://www.wider.unu.edu/sites/default/files/rp2006-101.pdf

Social capital in (dis) similar democracies: The development of generalized trust in Japan and Switzerland M Freitag - Comparative Political Studies, 2003 - https://www.researchgate.net/profile/Markus_Freitag2/publication/249699482_Social_Capital_in_DisSimilar_DemocraciesThe_Development_of_Generalized_Trust_in_Japan_and_Switzerland/links/56ebc04e08ae9dcdd82aff7a/Social-Capital-in-DisSimilar-DemocraciesThe-Development-of-Generalized-Trust-in-Japan-and-Switzerland.pdf

Gert-Jan Hospers, Beyond the Blue Banana? Structural Change in Europe's Geo-Economy, Intereconomics (2003) 38: 76. https://doi.org/10.1007/BF03031774 , https://www.researchgate.net/profile/Gert-Jan_Hospers/publication/23730517_Beyond_the_Blue_Banana_Structural_change_in_Europe%27s_geo-economy/links/554f17e708ae93634ec7112d/Beyond-the-Blue-Banana-Structural-change-in-Europes-geo-economy.pdf?origin=publication_detail

Ireland and Switzerland: The Jagged Edges of the Great Inflation, Working Paper 2006-016A by Edward Nelson, https://research.stlouisfed.org/wp/more/2006-016, 

Why Are Some Countries Rich and Others Poor? by Scott A. Wolla, https://research.stlouisfed.org/publications/page1-econ/2017/09/01/why-are-some-countries-rich-and-others-poor/