Because he correctly believed a two-front war would be problematic for Germany. The invasion of the Soviet Union started after his flight but he did know it was planned.
Germany’s war goal was primarily in the east and peace with the British would make it a lot easier to achieve. They wanted Alsace–Lorraine back from France but there was not really an idea to make the western countries a part of Germany. Look at https://en.wikipedia.org/wiki/Generalplan_Ost for the plans in the east that were a bit simple replacing all the population there with people from Germany.
The war against the Soviets would go better if they did not need to defend the Western Europeans at the same time. Add to that all the lend lease from the UK and US. If UK was out of the war it is a lot less likely US would support the Soviets.
A lot better question is why he believed that he could manage to broker a peace deal. I have no idea why believe it was possible.
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