a) The investors may have been given a different answer as to how much Reddit cares about making money, even in the short term (I don't mean that they were mislead, but that it would be bad PR to tell the Reddit community the opposite)

b) They may have a plan towards profitability over a certain period of time that they haven't shared with you or me but that was convincing enough to potentially be a good investment (or maybe other scenarios, such as selling to an even bigger company who already can monetise... such as Google)