LoP is just lucky. Location, location, location.
If you have the Dungeon Master Guide, you might want to take a look at page at the Outer Planes picture. See that infinite pillar? That's the Infinite Pillar.
On top of there is Sigil. That in itself is giving the deities a hard time to get in.
There are more reasons, as well.
First off, most deities simply aren't interested in taking over Sigil. Those who are, might want to take a trip through the portals. They'll soon after find their enemies doing just the same thing; walking through the portals from their own home planes. There will be a raging battle between alot of parts then. Since most lawful planes are smaller than the chaothic (excluding Limbo, which is only one plane), the battle will be between the smaller, but more "loyal" armies and the other armies, huge lumps of Tana'ri and other guys, all bashing at each others heads (or whatever limbs available). Eventually, they'll tire of killing themselves, and go to the nearby town, which is the third reason there are so few deities who could ever take over Sigil.
[url="http://www.planescape.com/adnd/outlands/narcoss/narcoss.htm"]Narcoss, the City of The Gods.[/url]
Additionaly, there are the Spell Keys, which makes magic stronger in some Outer Planes than in other planes, and so on.
Personally, I think that if someone could unite the Abyss, which is about 76 times larger than Baator, he would be the strongest entity.
Anyway, I suppose "Galand's master", the Lord of the City of the Gods, is the strongest person already.
[ 06-21-2001: Message edited by: RPgeek ]