Why did Europe need more raw materials during the 1800s?

"Europe" as an abstraction does not actually "need" things. The growth of specific political-economic interests in particular nation-states located in the region generated additional growth and expanded industrialization. The "age of revolution," in Eric Hobsbawm's words, paved the way for the "age of empire."
As the standard of living rose and population increased, along with changes in class relations, full-fledged capitalism largely replaced the mercantilism of the early modern period. A more educated, literate populace was aware of and coveted consumer products.
The ratio of cultivable land to population in Europe was already fairly low. Cultivating agricultural products in the colonies--both staples and, even more, luxuries such as sugar and coffee--increasingly served to feed Europe's people.
Similarly, expanded clothing and textile production led to cotton plantationing, as well as desire for dyestuffs. The creation of demand among the middle class increased the market for high-quality goods--what Thorsten Veblen called "conspicuous consumption"--including the antecedents of the modern fashion industry. Enlightenment principles of universal "rights" stimulated popular desire to display their equality through the goods they wore and used.
Weaponry for military expansion, railroads and related transportation infrastructure, and construction of the factories themselves depended on relatively low cost, steadily available metals and fuel--all in short supply in Europe.
In addition, continued exploration created its own expanding requirements. Britain's navy and vast commercial sailing fleet, for example, demanded huge amounts of timber and miles of cotton sailcloth. The virgin forests of New England were the ideal source of tall, straight trees for the masts.
Because capitalism was a well-integrated system that supported imperialism, demand for all types of raw and semi-processed materials fed into that self-perpetuating system--one whose success depended in part on the idea that "desire" was really "need."
https://books.google.com/books/about/The_age_of_capital_1848_1875.html?id=H3VsAAAAIAAJ