According to my knowledge of customs and the fact that IP winny (along with all other IP products) come from over yonder, it is a safe bet that the supply is dwindling and the delivery rate is being reduced due to a bottleneck at customs.
Besides, the powder can be found domestically (because it is being manufactured domestically in N. America).
Winny tabs are easier to take since you know exactly your dosage. I tried the powder form and I'm pretty sure that some days I was taking some mg more or less than others....