But the actual answer here is probably a combo of a few things: One, running a company is probably not as much fun as building a company. Much of my career has been "pioneer" roles where nobody else has done the job before. At a certain point, the foundation is laid and the problems to solve are different and often less interesting -- at least to me. It's the build vs. maintain thing.

Two, they started with good and noble intentions. Money got involved. A lot of money got involved. The noble intentions were replaced with reality.

Three, have you met users? As a site grows you have to deal with more and more people and people can be very demanding and not very appreciative. Coupled with the previous factors, I think original founders get burnt out and decide to take the cash and move on. The allure of building anew is too much, the grind of maintenance is too much, and the cash is too good to pass up.

Also four... there's a peak for any site. You often don't know when or how, but you do now that someday your site's maximum value, interest, participation, and all that is going to peak and then decline. Sticking around to fight the good fight may just mean passing up a payday and being left with a declining property nobody wants anymore.