According to this interview with the creators, it was too expensive and the audience reach didn't justify the expense.
..but it wasn't enough to guarantee a second season. Netflix chose to
cancel production on the grounds that it was too expensive and had not
reached a relevant audience enough to maintain itself. The creators do
not hide their disappointment and are still trying to understand the
impact of the decision.
"The cancellation of “1899” changed the way we think about our projects
a lot. Things have changed a lot in the industry and in society,
especially in the way people produce and consume content. We really
believe that if “Dark” were released now, it wouldn’t have any chance
— laments Odar. — We have so much content in the world today that
people are always consuming things quickly and without dedication.
With the cancellation, we had to think about whether our storytelling
is something people still want and whether we should adjust it.
Honestly, we don't have an answer yet."
Creators of 'Dark' and '1899' talk about new series and accusation of plagiarism: 'it was one of the most painful moments' (Google Translated from Portuguese)
As regards the audience figures, although it had good opening figures, the "completion rate" was appalling, with less than a third of Episode 1 viewers sticking with it until the final episode.
Netflix doesn’t share this info with the public, but according to data
analytics company Digital i (via What’s On Netflix), 1899’s completion
rate was just 32 percent.
The secret Netflix metric that got 1899 canceled — and could ruin TV forever