Is this because I answered before the bounty started?
Yes, bounties will not be automatically awarded to answers posted before the bounty started. The underlying assumption is that whoever posted the bounty was looking for a different answer than the existing ones, otherwise they would have awarded the bounty manually.
See How does the bounty system work?
What is automatic awarding?
Twenty-four hours after the end of the bounty period, if the bounty starter has not manually awarded the bounty, an eligible answer can be automatically awarded half the bounty amount. The criteria for an answer to be eligible for automatic awarding are as follows:
- The answer must be given after the bounty was started
- The answer must have a score of at least +2