There are no concrete written records as to exactly when and why it happened. There are strong theories, but it is still mostly speculation. Here are some you might enjoy:

1. The issue most likely arose due to the printing press. Less capitalization meant more efficient printing (you'll need to look up how printing worked in the 19th century, before typesetters like the Linotype machine, for this answer to make complete sense). Also, the metals used in printing presses were rare and expensive (all lowercase meant less types to replace, smaller orders from typefounders).
2. German has a longer tradition of capitalizing every noun, perhaps that's why they were the only ones who kept it. English, while Germanic, was still influenced by any other languages. In addition to that, British also published Celtic, Danish and French, which is another reason why the capitalization fell off earlier.
3. Another reason might be French influences, those guys do not capitalize common nouns! French used to be the lingua franca, after all, and it's certainly possible that it also influenced the capitalization of nouns.

These are the main theories, but a highly profilic writer (esp. one who despised the Germans and appraised French literature) could have advocated a more French spelling and influenced literature.