This probably will make sense to most people but to those who don't understand, allow me to explain. The whole concept of idea generation stems from our brains ability to be in constant flow. When our mind thinks creatively, there is an interaction between the outside world and the internal processor which is our own unique thought process.
At every second, millions of thoughts run through our minds and it becomes extremely difficult to comprehend it all but focusing on one specific thought at a time is what keeps the juices flowing. Trying to process all of your thoughts at once creates havoc and ergo does not allow us to keep going but instead burn out.