While most of the unis/colleges I worked with (10 years) would renew FTs' as early as January and never later than May, and begin recruiting new teachers (if needed) beginning around April, this is not typical of a lot of schools.
Many schools don't start talking to their current FTs about plans for the next year until like June, and they don't have an idea of how many FTs they'll need until that happens. So, they can't start the recruitment process until this time of the year, which makes it hard for teachers not currently in China who need to process everything and get visas.
The better schools tend to have FTs that stay on for years on end. So they don't have so many openings. Also, a number of schools don't like to deal with recruits they know nothing about -- they often will ask their current FTs to recommend people they think will be good fits. They also depend on recommendations from other unis -- maybe one of their graduates is working at a different uni with a FT who is really good -- so they might recommend that FT to their Alma Mater. Most unis prefer to hire teachers already in China -- they really like to be able to inverview in person, and check out the person's reputations from other places where they've taught in China.