>>7210462MSIA's were too small for some, that's why people bought gunpla over it. Also you are free to customise your gunpla in any way you want due to it not being assembled which is a huge turn on to those who want there own colourscheme or just want to make the own custom through a kitbash. Also, there is a certain amount of joy that comes from building your own figure that you just don't get with transformers. Finally, gunpla is pretty cheap when compared to transformers/robot damishii (even including shipping costs from Japan) due to Bandai rarely ever under stocking & doing frequent restocks of there older models unlike Hasbro/Takara who will charge a premium for older stuff with slightly different paint & choosing to under stock stuff which leads to either going through a middleman or waiting for a KO.
