Because it will cost them a certain amount to run another world, and if it isn't as long, they won't get so much money out of each user paying premium.
Say 1000 accounts play a world for 2 years, and out of those 800 accounts have premium the entire time. That is 24 months time 200 premium (about $5 a month?). So $120 over 2 years they make from each account, times 800 accounts is $96,000.
If it is a 3 month world, the same amount of of accounts buying premium would make them $12,000. Which is a hell of a lot less than they would make for a single world that is a lot longer.
You have to remember that Innogames is a primarily a business-based company. They need to make choices that get them as much profit as possible.