The brief answer is that Holland has long been the economic, cultural, scientific, demographic and military centre of the Netherlands.

It is the location of cities like Amsterdam, The Hague, Rotterdam, Haarlem, Leiden, Hoorn, Delft, and Gouda. In the early Medieval period other regions and cities wielded considerable influence (notable examples being Utrecht and the Frisians before that). At the beginning of the revolt against Habsburg Spain in the last quarter of the 16th century, many participating Flemish cities like Bruges, Ghent and especially Antwerp were still wealthier and much larger than the northern cities.

However, from the 15th and especially late 16th century onwards Holland experienced an unprecented economic boom. Helped by refugees fleeing the Spanish-occupied southern/Flemish cities, proto-industrialisation, newly reclaimed land, new forms of economic organisation (many trace the roots of modern market capitalism and the liberal political ideology to this period) and a variety of other factors, Holland and especially Amsterdam became a global hub and one of Europe's main financial and scientific centres. I suppose one could go as far that the Dutch Golden Age was in various ways almost a "Hollandic Golden Age". Holland accounted for well over half of the tax income of the newly independent Seven United Provinces of the Netherlands / Dutch Republic. It was the dominant voice in deciding the state's foreign policy in a period where the different Dutch provinces still functioned largely autonomous from each other in most ways (e.g. with own admiralities, legal systems, currencies). Ships and international trade often came from/went through ports in Holland, with less direct exposure for more inland provinces like Utrecht, Gelre, Drenthe and Overijssel. Mind you, through the Zuyderzee most of these could still trade, but they were just more agricultural and less urbanised. Locke, Descartes, Spinoza, Huygens, Grotius, Rembrandt, Vermeer and Hals all spent most if not all of their time in the Netherlands in Holland, not in, say, Drenthe.

This continues to this day. The modern provinces of South and North Holland still make up almost half the population, and is home to vital centres like Schiphol Airport, the port of Rotterdam, the seat of the Dutch government, the Zuidas financial hub, the media hub of Hilversum and the universities of Leiden, Delft, Rotterdam, and Amsterdam.

This influence ensured that "Holland" became synonymous to the Netherlands abroad, were people first met with traders from cities like Amsterdam, Hoorn or Haarlem. It is not unlike how "England" became a pars-pro-toto for the United Kingdom, a "yankee" from New York as nickname for Americans, or how names for Germany are often dependent on which Germanic tribe neighbouring countries were familiar with.

To read more, I would recommend James Kennedy's "A Concise History of the Netherlands", or Jonathan Israel's "The Dutch Republic" as sources.