when you say "all of these are a good idea" you are stating and emphasising that each individual separate idea is a good idea, in its own right.

When you say all of these are good ideas, you are placing the emphasis on all the ideas in the group of ideas just mentioned. You are saying they are all good ideas.

Take a look at this video

The first hours class by a teacher In south Africa, she is very patient you can watch the class as often as you like.It is a fairly comprehensive lesson on English Articles it deals with what is needed here.

Because that’s just the way the phrase is.:) however, you might actually say “is a good idea” and not “are”

You could also say "All of these are good ideas"

I think it should be "ideas" rather than "idea", which would not require the "a"

"all of these are good ideas"