Because grammar calls for a comma after an introductory prepositional phrase but does not require it. For extremely short prepositional phrases where a comma creates a pause that no one thinks or says, you probably shouldn't put a comma. The longer a prepositional phrase gets, the more recommendable a comma becomes. Still, all of that is based on one's subjective opinion and not clearly stated rules of grammar, making the comma after an introductory prepositional phrase a question of style, not grammar.
Here are some quotes from what various grammar sources have said about it:
A comma may also set off a single prepositional phrase at the
beginning to make the sentence clear.
Note the use of "may" in the above.
When a prepositional phrase expands to more than three words, say, or
becomes connected to yet another prepositional phrase, the use of a
comma will depend on the writer's sense of the rhythm and flow of the
Notice how it says above that the use of a comma will depend on the writer's sense, meaning it is subjective, a personal preference, not a hard-and-fast rule. Also, notice how it interjects "say," meaning "for example," so the aforementioned "three words" isn't actually a bona fide rule but only an example of the number of words in a prepositional phrase one might use as a rule of thumb for putting a comma afterwards.
When an introductory prepositional phrase is very short (less than
four words), the comma is usually optional.
Pat attention to how the above says "very short" and "usually" and doesn't actually say "is less than four words" but only includes "less than four words" as a parenthetical elaboration. All of that points to the employment of that comma as being optional.
All of that said, I'd have definitely put a comma after the introductory prepositional phrase "after the end of the first year." I think most grammarians would. But the writer can't actually be faulted grammatically for not putting one.
Also, do keep in mind that grammatical errors abound, even in books. Don't just assume just because you're reading it in a book or a magazine or some other publication, that it's necessarily right or necessarily what is even recommended.