It's because that is the syntax. This is the syntax for creating a Scala ArrayBuffer:
var fruits = ArrayBuffer[String]()
var ints = ArrayBuffer[Int]()
The key thing to know is that the keyword new is not required before the ArrayBuffer. (This is because ArrayBuffer is either defined as a case class, or because it has an apply method defined. I haven’t looked at its source code to know which approach is taken.)
While I’m in the neighborhood, here are some other ways you can work with ArrayBuffer:
val x = ArrayBuffer('a', 'b', 'c', 'd', 'e')
var characters = ArrayBuffer[String]()
characters += "Ben"
characters += "Jerry"
characters += "Dale"