Well, "we" did. It just took longer than most people thought. Segwit was put in last year, it just hasn't activated yet.
Why did it take so long to develop? I think it's a combination. Many people wanted to get it right, a bad solution could be worse than no solution. There was also concern about miners blocking it, as many had been very skeptical about lowered fees.
Why does it take so long to active? Yeah, this is the tough one. It was deployed the same way as other soft forks. But this turned out to be quite contentious as it played right into an ongoing governance conflict. Worst case, the current deployment has to time out later this year and then it can be safely re-done.
Why did it take so long to develop? I think it's a combination. Many people wanted to get it right, a bad solution could be worse than no solution. There was also concern about miners blocking it, as many had been very skeptical about lowered fees.
Why does it take so long to active? Yeah, this is the tough one. It was deployed the same way as other soft forks. But this turned out to be quite contentious as it played right into an ongoing governance conflict. Worst case, the current deployment has to time out later this year and then it can be safely re-done.