There were nearly 300 open tournaments just sitting there, stagnating, and they were never going to start. This is a bad experience for everyone involved -- tournament creators would create a tournament and never get to play in it or see it start. People joining tournaments would come to the page, and either get overwhelmed with choices or they'd see one they wanted to join, join it, and then never get to play.
We want all open tournaments to start within a reasonable amount of time. For this to happen, the ratio of people creating to joining tournaments needs to be reasonable. For example, for each person that creates a 500 person tournament, there needs to be 500 people willing to join tournaments. (1:500 ratio). This wasn't the case before, clearly, as evidenced by having tens of thousands of unfilled tournament seats.
Due to this Fizzer decided that you shouldn't just be able to create open tournaments. They now cost coins to make. If my memory serves me right there is also a limited amount of open tournaments that can be open at the same time (but not 100% of this).