An ArrayBuffer would have to be fully read before it could be used, and all of the data would have to be in memory at the same time. In contrast, a Blob can be read as a stream, avoiding having to have all the data in memory before you can do anything with it (or at all).

For many purposes, incremental processing of the data from a stream is what you want (such as playing audio or video).

For some purposes, you really need all the data before you can do anything with it, in which case you might use an ArrayBuffer.

As for why Blob is the default, I'll speculate that it's because it's more flexible. When you need incremental processing, you can get it via the Blob (using a stream) without waiting for all the data, and if you need all the data before you start, you can always get an ArrayBuffer from the Blob via its arrayBuffer method.