i dont know.
but my guesses:
thats where the jews were
thats where it was easiest to do
the largest area they could keep their “prisoners” there
largest camp
place where they could make a large camp

id look them up if i were you.

Many were in Germany, so I think is was a transportation issue.

Poland was one of the first nations to fall under the heels of Nazism and the German’s infrastructure was more easily put in place due to the proximity/adjoining of the two nations ;good luck !
:0)

The reason you give, although it can be put better, explains one of the critical factors. The Nazi command did not want the ordinary German to realise what the Final Solution actually entailed, this had the knock-on effect of keeping the details of the camps from leaking out to the rest of the world until later in the war.

Most of the jews were in russian and poland. Some of the western european countries had some control over their countries and had sympathies towards the jews. Meanwhile poland and russia were under direct germany military control so they could round up all the jews they wanted. The germans begran rounding up jews from the outset in poland and russia but didnt start rounding up jews from other countries until the holocaust was in full steam and that was later in the war.