Because parsers, like humans, read from left to right. Important parsing decisions are usually made as early as possible. An example is recursive descent parsin

Because parsers, like humans, read from left to right. Important parsing decisions are usually made as early as possible. An example is recursive descent parsing, which derives its simplicity from making decisions first, allowing a top-down parse. And when humans skim text, I bet it can be shown that it's easier if the syntactically significant locations are aligned near the beginning of each statement.