1.) Less inventory. Advertisers bid for spots on the platform, which means that if there're fewer users accepting advertisers, the remaining spots will require more BAT, which requires purchasing more BAT on the open market. Higher demand + constant supply = higher price.

2.) Fewer user rewards. Users receive BAT when browsing, which they can either tip to creators or sell on an exchange (I'm not sure the latter option is available right now). Unless the users or creators are themselves advertisers, there's no use for BAT though, other than selling it to an advertiser. So when fewer users are receiving BAT, there's less available for purchase on exchanges. Constant demand + reduced supply = higher price.