I believe it's to do with the exponential growth of the STEEM blockchain. How Steem Dollars works is similar to the concept of bitshares. Imagine I have a box and I want that box to be worth 1 USD. Inside that box I might put 5 things worth 20 cents to make that 1USD.
However, that object fluctuates in value due to supply and demand and just how the market works, so say it hits 10 cents, we will check the value of the object and determine how many more objects to put in or take out and in this case, they add 5 more of those objects to make 10 total objects worth 10 cents each.
That box, is supposedly worth 1USD all the time. Currently, as the growth of Steem has been so unprecedented, it's each 'object' is worth more than that dollar value, thus if u put one of those objects in the box, it exceeds the desired value of one dollar and the minimum number of objects you can put in is 1 (apart from nothing, which would make the value zero). Hope that makes sense.
Keep in mind that it's not Steems fault, it's just their product is so good in in such high demand that the measures to maintain a stable value couldn't keep with with their exponential growth in size and user base.
