Maybe the chapter has internal problems and they want to work on them first before bringing in a huge pledge class. Sometimes it's hard to get things like that worked out when you are worrying about the girls who weren't 100% into you, but were only 75% into you and you have to keep rushing them after they receive their bids. Just a thought.
I mean, a lot of us have had that experience of a girl at pref party who only wants A sorority, not YOUR sorority, and knowing we have to bid her because our butts will be in a sling if we don't make quota. And knowing we're going to have to babysit her for the next 6 weeks so she doesn't depledge. Get too many of these in the sorority, and people will self-terminate and numbers will go down anyway.