Relevent conversations: Bubble World Theory & Myth-Building, The Amazon Bubble.
By virtue of its horror genre or its Vertigo or proto-Vertigo status, Swamp Thing was probably one of the DC Universe's first Bubble Worlds. Yes, he still interacted with the rest of the DCU, but existed in his own realm, with its own rules, and indeed, added to the sum of the DCU by creating a new and interesting corner of that universe. Swamp Thing's Bubble has always been leaky: The DC Universe isn't so much a soft drink full of tiny rising bubbles as it is a lava lamp with bubbles flowing into each other.
On the one hand, Swamp Thing started the ball rolling for Vertigo's cloistering. Alan Moore introduced John Constantine in those pages, and John has welcomed a number of titles (Sandman, Books of Magic) into the Vertigoized DCU, thematically leading to more. But Swamp Thing's leaked into the so-called normal DCU as well. When Moore changed Swamp Thing's paradigm to make him an earth elemental, a guardian answerable to the Parliament of Trees, he created the need for other elemental guardians. Soon, Firestorm was named fire elemental, Red Tornado air elemental, and a creature called Naiad became water elemental. Aquaman would eventually take over that mantle, and there was even talk of Captain Atom as a quantum elemental. Swamp Thing's "rules" had become DC's "rules".
But there's another important concept that was part of Swamp Thing's bubble, and that's the Green. The Green can be described as a dimension that connects all plant life - part teleporting matter stream, part collective consciousness (manifested as the Parliament), part dreamtime. It might have stopped there, but it got a mythological boost when the Green went to war against the Gray, which represents all fungal life, and in a sense the decay of plant life. All of a sudden, the DCU is overlaid with more than one nature-based continuum. Jamie Delano's take on Animal Man, for example, made him draw his powers directly from the Red, representing all animal life.
In the final months of the series, pre-WTF Mark Millar, possibly working from Grant Morrison's notes, took all this to its logical extreme by having Swamp Thing meet and become the champion of other Parliaments: Stones, Waves, Vapours and Flames. The Parliament of Stones, for example, exists in the Melt, and so on.
When he is of all elements, he essentially becomes Gaia herself.
But wait, it doesn't stop there.
The Parliament of Worlds. Or enlightened Worlds, actually, which seems to mean planets where life has evolved. Perhaps our own consciousness is a product of our world's own evolving consciousness. Represented in the Parliament is Earth, Mars and Oa (the others are not named). And unfortunately, that's the last issue of the series, offering a development that creates a fine permanent ending for the character, but sadly abandonned when he invariably made a return. The bubble has burst.
But why? Swamp Thing became too powerful? I could easily imagine a Swamp Thing book built like Vertigo's other star, Sandman, with the various natural continua as the Dreaming. Alec still tries to live a "normal life" with Abby, but is embroiled in fantastic stories as Earth/Gaia is undermined by evil forces. There's political maneuvering among the various Parliaments (think of Swamp Thing as President, the Parliaments as States and the Worlds as the UN). It opens up a ton of stories on the macro scale too as, say, the Worlds get together once a century to honor their fallen comrade Krypton.
In the end, perhaps Swamp Thing's Bubble World became to ambitious to be allowed to leak out into the DCU.