I'm wondering myself what keeps the BTS experience still fresh and exciting as I surely must have played over two hundred games (not that much right now, but lots of it when I was unemployed). These forums are a big part of the appeal the games has right now for me, as there's always some new information to gain (after all these years!) or some new map to try and beat.
What keeps it fresh and exciting is that there are so many different options and things that can happen every time you play.
Just starting on a new random map and exploring the world is enough to keep coming back IMO.
And with different maps, you get different games.
In one game you might have tons of room to expand and have to worry about how you are going to afford your empire. In the next game maybe the AI boxes you in early and you only get a few cities, so you have to worry about how you can get more land.
In one game you might have plenty of resources and be able to build whatever you want. In the next game maybe all the good resources are in someone else's land so you have to scheme how to get them.
In one game you might find yourself in barren tundra and have to work on growing your cities all game. The next game you might find yourself with tons of flood plains and have to figure out how to keep your population from growing too fast.
In one game you might have to work on building your economy while the next game you need to work on building your army.
There are just so many possibilities and challenges that it never gets old.
I just played a game recently where I started out in a tundra region, yet had access to like 10 fur resources by the time I expanded and settled my permanent borders. There were virtually no other fur resources anywhere else in the world, so I had a monopoly on trade. That was very interesting, I've never been in that situation before. I was basically able to run 100% research the entire game because virtually every civ was paying me gold per/turn for fur. I've had this game for a long time and that was a first for me!