While I more or less agree with The_Warlock, Death is already present, etc.
I would like to know what the "bunch" of other reasons are.
Some things in games are limited, because with no limits things usually get out of control.
Basic mechanic of the Reaper is: when you roll a 1 for your move, you may move the Reaper by rolling 1 die at the end of your turn.
In the Inner Region, you don't roll a die for your move. With Dragon Tower, things get even more complicated because movement is increased by defeated Enemies and happens at the end of the turn.
Why should the Reaper figure not be allowed in the Inner Region?
1) How would you move the Reaper in the Inner Region? Will you move it like characters? Will you roll the die regardless of normal movement rules? Why should the Reaper break normal movement rules in the Inner Region while he doesn't in any other Region?
2) Characters in the Inner Region don't roll a die for movement and can't move the Reaper away from them. Is it fair that characters outside the Inner Region can move the Reaper to characters in the Inner Region, and not viceversa?
3) What if the Reaper comes to the Crown of Command? Does he stay on the space and automatically encounter characters every turn? Or every time a character rolls a 1 for his move? If he moves one space per turn and he may leave the Crown at will, he could be moved back and forth and land repeatedly on a character on the Crown? If he rolls the die, does he stop on the Crown and may not move further?
4) What if a character rolls a 6 on the Reaper chart in the Inner Region? May he teleport to the Crown?
5) In the Dragon Tower, does the Reaper move one space per turn, roll the die regardless of any movement rule, or does he draw Dragon cards?
Of course there's a house rule solution to all of these questions. If you bother to find it out, you'll be fine with your game.
I play with 100% Rules As Written, not because I like every rule, but just to have minimum rule debate during play. The Reaper Rulesheet was written in a literary form, but I think it's quite clear in saying that Reaper in the Inner Region should not be allowed. Above there's a bunch of reasons, you evaluate if they're good or not.