There is one big reason.
Religion has no small amount of fear and blame and, if you are Catholic, some guilt. Environmentalism has those also yet they don't have 5 billion members the way religion does because they lack the one important thing: Religion offers salvation along with the guilt.
Environmentalism doesn't, at least not yet - 'be a wealthy elite who can afford solar power and organic food' isn't really a road map to forgiveness everyone can follow - but once they figure out a real salvation solution rather than just bouncing from fundraising cause to fundraising cause, all the other pieces are there for Gaia to take her place among true religions.
Currently Gaia already has:
A Prophet, in Rachel Carson. Like Jesus, Carson was married to work rather than another human being and and she also came from a common background, Chatham isn't even in the top 5 schools in Pittsburgh, which helps environmental elites assuage their liberal guilt. Like with Jesus, miracles are attributed to Rachel, though her actual contributions during life were met with skepticism.
About her most important work, Silent Spring, science was derisive, but there has always been derision from science about miracles. Professor I. L. Baldwin, professor of agricultural bacteriology at the University of Wisconsin, led the National Academy of Sciences committee studying pesticides and wildlife and reviewed the book in Science. He called it a “prosecuting attorney’s impassioned plea for action.” In 2015, environmental groups primarily consist of attorneys so that is a compliment to them, but in 1962 it was about as insulting as a scientist could get.
After death, she also became reborn, as an inspiration to millions, with entire sects all claiming Apostolic succession from her. Like Jesus, details of much of her private life do not exist, she and Dorothy Freeman destroyed hundreds of their private letters to each other before she died. People like enigmas.
A Judas, in Dr. Patrick Moore. He was a co-founder of Greenpeace but now they deny his very existence because he was seduced by a universe full of natural laws and unreasonable optimism that humans are not parasites. He didn't get his bag of silver, though if you want to be told he is a shill for modern day overlords like Monsanto Gawker bloggers will do so to achieve their daily pageview quota.
A Bible. The defining work of a religion is long on storytelling and short on evidence and Silent Spring has that in spades. Like with The Bible, it has acolytes who still find ways to map what it does have to their cultural topology. If you want to believe people sprayed a chemical in their basement and then died from it a few months later and just need it to be written somewhere, Silent Spring is the way to go, just like The Bible tells us that St. Peter healed a beggar after Jesus died.
Soldiers Of God. These are needed to do the dirty work. For men, these Soldiers will be the types who will meet under the auspices of groups like the IUCN and plot to get something banned by foregoing studies and instead rigging the journal system, which will mobilize the faithful to take action. They will give up their integrity. Women are expected to give up their clothes. Given the nature of the 19th century-style everything that environmentalism prefers it is no surprise that environmentalism has the highest wage gap between genders; they won't let women be priests, but they can be sexy nuns. It just doesn't pay as well.
Evangelicals. The great thing about both scientists and people who hate science is the belief that the science is actually on their side. While that thinking makes total sense for scientists, it takes a special sort of conviction for environmentalists to rationalize that scientists are wrong about GMOs, vaccines, and energy but super smart when it comes to global warming. Not just anyone can make that argument with a straight face, and because it takes a different kind of person to become a true evangelist, they first must be...
Guided on the righteous path. Conviction doesn't just happen, it must be nurtured in a supportive environment. People will have doubts that science is wrong on everything except global warming so they have to be managed through the conversion process carefully; smarminess and condescension can be used against scientists and other heretics but for those who will be brought into the flock, a different technique is needed. Questions must be answered with properly framed answers in which only the current environmental talking points can be true, the cultural landscape must be shown to be positive and essential. Once they are steeped in the belief that they are doing Gaia's work, the tipping point will be when they publicly express their commitment. Then they will be expected to annoy their friends as a central part of their activism. "No one ever quietly went on a cleanse", the saying goes, and that includes flushing evidence and reason out of their brains.
The Salvation Solution
In most religions, you can't just get into heaven by doing good works. If you could, it would be possible to be a real jerk and just show up at a food kitchen once in a while and still ascend accordingly. Likewise, environmentalists have to accept that sending money to pay lobbyists to get more and more restrictions on business passed, or filling out an online petition after shopping at a farmer's market, isn't really going to get the job done.
Obviously a good first step would be to separate real concerns about complex science versus manufactured doubt. Protesting Golden Rice, for example, is not only silly and lacks any evidence basis, it would send environmentalists up on criminal charges at The Hague if international law had any meaning. Fetishizing solar and wind in an Anything But Oil haze is also not doing anything for the environment - but nuclear energy would.
What needs to happen is for environmentalism to get back to science, the way Norm Borlaug and Rachel Carson saw it as the future. Rachel Carson wanted fewer pesticides and that has happened. Better usage of the land and modern science has prevented enough soil erosion to fill more than 15,000,000 dump trucks.
Gaia would be pleased if her acolytes would make that kind of difference instead of just asking for money all of the time.