to add on to this question, why was vanilla chosen as the default "basic, boring" flavour? according to wikipedia, it's actually one of the most expensive spices.

Before the 19th century, both ice cream and vanilla were luxuries of royalty and the very rich. Two inventions changed all that: insulated ice houses (around 1800), which allowed the commercial production of ice cream; and the method of artificially pollinating vanilla orchids (developed around 1840), which allowed the large-scale cultivation of vanilla.

It's difficult to give a definitive answer to your question, but it's likely that both ice cream and vanilla retained their cachet as luxury items long after they became available to the general public, and that the flavors of cream and vanilla were complementary enough that people began to associate them with each other. Even today, the word "cream" is often shorthand for vanilla cream: cream soda is an example that jumps to mind.

A great book on this subject is "Vanilla: The Cultural History of the World's Favorite Flavor and Fragrance" (Rain, Patricia: Tarcher, 2004).