Sooner or later, tribes begin to exclude interested but unaffiliated newcomers.

It happens to religious sects, to surfers and to online communities as well. Nascent groups with open arms become mature groups too set in their ways to evangelize and grow their membership, too stuck to engage, change and thrive.

So much easier to turn someone away than it is to patiently engage with them, the way you were welcomed when you were in their shoes.

There are two reasons for this:

It's tiresome and boring to keep breaking in newbies. Eternal September, the never-ending stream of repetitive questions and mistakes can wear out even the most committed host. Your IT person wasn't born grouchy--it just happens.
It's threatening to the existing power structure. New voices want new procedures and fresh leadership.
And so, Wikipedia has transformed itself into a club that's not particularly interested in welcoming new editors.

And the social club down the street has a membership with an average age of 77.

And companies that used to grow by absorbing talent via acquisitions, cease to do so.

This cycle isn't inevitable, but it takes ever more effort to overcome our inertia.

Even if it happens gradually, the choice to not fight this inertia is still a choice. And while closing the gate can ensure stability and the status quo (for now), it rarely leads to growth, and ultimately leads to decline.