I guess nothing wrong with the minimum deposit they set. Maybe they are not targeting those lowbies and trying to deal with people who can deposit on the minimum amount they require..
Maybe they are not targeting the lower class. But there are many countries where $15 is a lot, in my country $15=BDT 1850+. So according to my country it would be nice to reduce it a bit, there are many gamblers who gamble for fun and don't deposit many dollars. So it is better to consider all aspects.
However, this time they plan to run the signature campaign for a few months, if they get good results they may decide on a longer term.