Because the components and PCB tracks are only rated to pass a maximum current of 400mA. If you try to draw any more, there's a risk it could overheat or some protection circuitry will kick in (the latter is more likely) and reduce the output voltage, therefore the current (Ohm's law).
To increase the current capacity you'd need to replace some of the components with ones with higher current ratings (this isn't possible without an understanding of how the circuit works) and widen some of the tracks on the PCB which would require a redesign of the PCB.
Oh and by the way, in future when starting a thread, please use a more descriptive title than just help. Consider what you need help with. There are many help threads here and there's more chance people will read it if the title gets their attention.
Thanks for your reply. Thought I might be able to make some minor changes that allow me to increase the output current to 500ma so I can use it to charge my iPhone 4.
Will also input a better subject next time. Thanks again!