It's all a matter of habit and consistency...

Why is it SO much easier to introduce a minority language at birth? Why is it so hard to introduce the language later? I guess, I've never seen a family (with older children) able to make a deliberate switch from speaking the majority (community) language to only speaking the minority (target) language at home. It's incredibly hard to wake up one morning and say, "OK, from here on out we will all only speak Spanish (German, Italian, etc) to each other." Why is this so hard?
I think it's because we are such creatures of habit. We don't like to spend much time thinking 'about' language. Language is a tool we use to communicate. When we pick up a hammer, we don't spend much time thinking about that hammer. We're spending our time thinking about what we're going to accomplish with the hammer. It's the same with language. We don't want to waste our time thinking about the words and syntax that we will use, we mostly are thinking about communicating a certain thought. And usually, we prefer to use the most efficient tool (or language) to which we have access.
The reason why it's so effective to introduce a minority language early, is because we establish language patterns and routines which become natural and efficient. If we've been using certain words and phrases for certain routines since birth, then each time we encounter that particular routine, it will trigger those particular words and phrases.
I've seen this in action with my older kids. We started off speaking only German to all of our children. As they entered their teen years, our family language had mostly switched to English (the community language). They retained their ability to understand and speak German, they just didn't use German much at home, with the exception of certain daily and weekly routines. Because the German language had been so ingrained in conjunction with those routines, they almost always used German in those situations.
The other reason it's easier to start out with the target language, is because it just makes the whole bilingual rearing so, so, so much easier. I think about my youngest three children. They have been speaking exclusively German since birth. I don't even have to make an effort to get them to speak German, they just do it. It's what comes naturally to them.
So, if anyone were to ask me for advice on how to to raise children in a non-native language, I would tell them to start at birth. That way, you're learning right along with the children and you're establishing all your family routines in the target language which makes raising bilingual children so much easier and sets you up for success!!