Because with the GPL you can still obtain the source code for any changes made to your original work if they want to put any software made for it public. GPL'd software is required to have the source available under GPL to anyone it is distributed to. You can merge any of this code back into your work, as it has to be GPL licenced.

With the MIT licence they can distribute proprietary software based on your work and not have the source available, as this is not required with the MIT licence. So you have no way to get the changes or improvements other parties have made if they choose not to give them to you.

Take what I am saying with a pinch of salt, others can give a better explanation. I'd suggest looking it up yourself.