As it has Surge Surfer, it is likely just to put another Hawaii-type spin on things and make it even more Alola-like because it's surfing. Having this unique ability, in turn, distinguishes it as an Alolan specialty.
Momo1234567890 also makes a good point that it's aboard its floating tail as if it were surfing. sumwun also makes an excellent point that the trend with Pokemon's developers seems to be avoiding new Pokemon having Levitate and another ability (though they already have Bronzor???) if at all possible, and Surge Surfer seemed like too good an opportunity to pass up, I guess.
You could ask the same question about Magnezone too, but there aren't as much explanations available for that one as there are Alolan Raichu.