8.5 is very high for an expansion for a two (or three?) year old game.
Agreed. As you will notice, Gamespot almost always rates a game's first expansion pack higher than its second one unless the first one was truly horrible. Most likely when Warlords was released Gamespot still played CivIV a lot and they must have thought "yay, one of our favorite games just got more content!", whereas for BtS they thought "oh, we've moved on to other things by now; this exp pack doesn't do much." Here are some good reasons why Gamespot rated it worse than Warlords.
1. They've probably moved on to other games by now.
2. The scenarios, while better gameplay-wise, have generally crappy ideas, weren't what most fans were hoping for, and worse still, some went, uhm, OFF-TOPIC. Seriously, isn't one of the main rules for video games to "stay on topic?"
3. It feels much less like Civ.
4. You're now pretty much forced to wage warfare in the modern age because the space race VC takes much longer now, and, worse still, there's no backdoor domination victory (in patch 3.13). This makes games slow down to a crawl and doesn't make it fun anymore.
5. The UB feature offered in Warlords added way more to the Civ experience than anything in BtS.
At least IGN rated BtS higher than Age of Empires II.