As far as i am considered, i exactly don't know what is the main reason behind this fact.
But, as far as i think so the main reason behind this fact can be that they may be want to get maximum amount of capital money from that particular trader. Gaining more and more profits from their traders may be their main policy in this business. And, not only in this business, but, in any other business this is the main policy of the brokers that they try to gain more and more profits from their affiliated traders and make the maximum amount of the accumulated capital amount.
Secondly, as far as i have some idea that if the trader has some good amount of the capital money, only then he may be able to earn some good profit and also withstand the loose if these are faced by them.
According to my point of view, these two are the main reasons behind this important fact that the some brokers have " High Minimum Deposit " Policy .............................