For a card counter, the primary advantage of fewer decks is that the effects of removal (EOR) are not as diluted. For example, removing a five from two decks has a greater effect than removing a five from six decks.
For a basic strategy player, the primary advantage from fewer decks comes from the slightly higher frequency of blackjacks.
With a single deck, the probability of getting a blackjack =
(prob of a ten) x (prob of an ace) x 2 (since you can get a blackjack by either getting a ten followed by an ace or an ace followed by a ten)
= 16/52 x 4/51 x 2 = .048265.
With six decks, the probability of getting a blackjack =
96/312 = 24/311 x 2 = .047895.
Another advantage of fewer decks is that your knowledge of the dealer's upcard and at least your hand's total represents a knowledge of a greater proportion of the total cards in use.