Cosmic microwave background radiation: why did it take so long to get here?
Here's a very, very simple explanation:

"why did it take so long to get here?"

There's two aircraft carriers, A and B, fifty miles apart on the ocean. You've got a helicopter that flies 50 mph.  Of course, it takes one hour to make it from A to B.

Now, imagine A and B are both moving - they're steaming away from each other at a good speed.

Of course, it now takes longer than an hour for the chopper to make it between A and B.

Regarding the many, many mysteries of the metric expansion of space, this trivial observation is just one thing to bear in mind. Note that of course this is not a full and complete analogy, and the analogy is indeed utterly wrong / totally irrelevant to many aspects of the broader discussion.

"I thought that the photons were "trapped" in an expanding ball" sure, the chopper can only move at 50 mph over the water underneath it meaning that at any given time it can only have moved so far over the water underneath it (a sort of "circle of achievablity", if you will).  But you can see with the carriers moving the hell all over the place, it's just an open question whether the chopper reaches B in much less than an hour, much more than an hour, or can never reach B at all.