The answer is simple: we have more users posting, which means more competition.
Currently there are 57600 Steem being distributed daily for posts and curation rewards, at 30 cents per steem that is 17280 USD being handed out to steemians every single day!
However we have more users and they are posting and voting more and more, here are some graphs and numbers provided by steemle (credits to 
A day like yesterday we had a record high of 2670 posts and comments (edits are also counted) and 10391 votes; this brings an average of 3.89 votes and 21.57 steem per post ($6). Of course we know some posts do better than others, so you could have gotten 100 bucks or 20 cents.
Now, does this mean that as we grow the rewards will always be less?
Well not necessarily, as we have more users there will be a larger demand for steem, if the demand is higher than the inflation we could see higher rewards.
I am currently quite optimistic about the growth of steemit. The site has been live for only a few months and we are already 111,701 on the alexa ranking, out of more than 1,050,020,000 websites, this is a huge accomplishment in a very small time frame.
So my advice to you is: Blog good quality content and vote strategically to earn as much SP as possible, because one day you could be a big happy whale!