I guess because the majority of people who saw it never signed up to steemit and voted?

Indeed that's the reason.
I was too blown away by the numbers of time it was shared on Facebook, it shows Steemit's potential to make post go viral on other social media websites too. Since Steemit has 7 day limit to reward payout that post is not going to make any money but it gives me hope to make huge amount of money on Steemit beside commuinty payouts. Might be affiliate links can be added in such posts later to gain some profit.