It's only been three months since that anime ended.
Remember these shows are broadcast in a limited number of timeslots in Japan, and the planning/negotiation/funding stage for each show is many, many months at the least. And everyone involved wants to know what's happening far in advance. Would you feel secure if your company couldn't guarantee they'd have any work in 6 months? Neither do anime studios, or TV stations. They want to know far in advance which shows are in the pipeline and what the timeslot is, they probably plan for at least 6-12 months ahead, probably a bit longer. So if something is a surprise hit, it's going to take until they reach the end of their planning cycle to fit it in, and even then, it has to compete with other productions that are being pitched.
Another issue is that the manga is still ongoing. There might not be enough material for a full second season. Looking over wikipedia, the anime covered the first 8 books of the story. At the time the anime started, there were only another 4 books released. So they had no way of knowing whether the next 8 books (it will be up to 16 books this year) would have been suitable for a full second season. And who in their right mind is going to pre-order an anime series for a series of books that hasn't come out yet? The TV station and the studio both want to know what exactly they're paying millions of dollars for. They're not going to make that sort of investment until there's enough new story to do a whole season.