Several reasons.
One, although there were some concentration camps (i.e. Dachau near München) in Germany, he didn’t want pure Germans to be “disturbed” by the death camps.
Also, Germany occupied Poland and had it so firmly under control, they could do what they wished.
Poland had the largest Jewish population.
Finally, good rail service.

Hello,
In a word, “Location.” He couldn’t do it on German soil. It would’ve warned the Jews, & mobilized international furor much earlier. He needed seclusion for his dirty work. Ostland (East land, aka Poland) was perfect for this. It is sandwiched between Germany, & the Russian frontier. You have to remember the technology back then. No one knew what he was doing for quite some time. He could resettle Germany’s Jews. Then, resettle Polish Jews w/little travel involved. After Barbarosa (Invasion of Russia), the Einsatz Gruppen followed behind the advancing Germans to continue the slaughter eastward. You should read some ore about it. It’s the epitome of evil.

Just to add to the answers before mine, Poland had the largest number of Jews in Europe by then.