1. Sigil is dominated by the Outer Planes. While Sigil has doors to everywhere, it's shaped most of all by Outer Planar thinking - specifically, the philosophy that is the source of so much of the politics and conflict of the Cage. It's the Outer Planes that make the Cage what it is, from its ring-shape to its government.
2. Sigil has doors to everywhere. There are about as many portals leading to the Elemental Plane of Fire as there are leading to the Astral or the Material Plane or Bytopia or the Far Realm. The multiverse, as seen from Sigil, is a ring, but that ring is defined by the shape of the Cage, not by the Outlands or the Ordial or the Material or anything else. That's not to say that its portals lead equally to all parts of every plane - far from it. There are few portals leading to the vacuum of Material wildspace, but more than one leading to Greyhawk and Waterdeep. There are more portals leading to the other gate towns than there are to any other part of the Outlands. Sigil touches every plane, but the way those planes affect Sigil is altered by all the other planes, the Outer Planes somewhat more than others.
3. Sigil is a gate town. You know how dramatically the people of Bedlam are affected by the proximity of its Howling Gate? And how strange the people of Automata have become thanks to its gate to Mechanus? The Cagers are affected every bit as much by its portals, only they have a million times more to deal with. These portals sometimes cancel each other out, but more commonly their effects are just localized and combine in strange, unexpected ways. Sigil is a million gate towns. Sigil is a crazy painter's palette that's never been cleaned.
4. Humans, second only to rats, are the most populous race in the Cage. This doesn't mean that the city is any more influenced by the Prime than any other plane - humans come from everywhere. There are thousands of different races in Sigil, maybe far more, but humans are the primary medium through which the city expresses itself. That's not to say that Sigil is a human city in any respect - it wasn't designed to be a safe place for humans to live, its streets and buildings aren't always built on a human scale, human thinking doesn't dominate the way Sigil works, and nothing in or about Sigil cares if there are humans there or not - but humans live there anyway, and somehow manage to thrive. There're probably a larger concentration of human-descended planetouched in Sigil than anywhere else, but pure humans still outnumber them by quite a bit.
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