Because there were more Jews in Poland than there were in Germany.
Hitler's wholesale murder of Polish (and Ukrainian, and even French, Dutch, and Greek Jews) makes nonsense of the present day Hitler apologists' claim that Hitler thought that Jews were 'enemies of the state.'
Obviously a Jew who had the appalling luck to live in a country invaded by Hitler could not be an enemy of the German state, responsible for treason in 1916 (even if it had happened.)
Even if Hitler had believed in the myth of WW1 treason he could not imagine that it gave him the right to persecute and eventually murder all Jews everywhere.
As for Midnight Run/dave's nonsense about typhus, ignore it. His claim that virtually all Hitler's victims died of typhus or other infectious diseases - and his other wicked contention that it was 'their own fault' because they and not their Nazi captors caused the filthy and crowded conditions in the camps and ghettos which made the spread of the disease so easy is nonsense as any sane person can see.
A small, but sad example proves this: Anne Frank and her family, although living in crowded and uncomfortable conditions when they were hiding from the Nazis, did not have typhus. When they were arrested (for the crime of being Jewish - there was no other reason - Otto Frank was a veteran from WW1 where he fought for Germany) and sent first to Auschwitz then to Belsen they died. Almost certainly of typhus. And those who sent them there are as responsible for their deaths as they would have been if they had been merciful enough to shoot them instead.
But there is proof of the gas chambers, which Dave ignores, and of the massacres by shooting, drowning, and the horrible 'death by agricultural implements.'
A conversation recorded amongst German POWs who did not know they were being recorded mentions the difficulty of shooting small children...they just wouldn't stand still.
This is the kind of thing dave is defending.
And questioner, please note Polish is one of those words that really needs a capital letter.