the short reason is that anemones require extremely stable water conditions and excellent water parameters. This stablility only comes with time. A tank needs several months to actually become an established eco-system, in fact, it is alway a work in progress. During this time the aquarist wil also need to mature and grow in the hobby. These are not considered easy animals to keep. By waiting until your tank is much more established, you just make things easier on you and the animal. This is hooby of patence. I read an article no too long ago, a link herre to another reef forum or wetwebmedia.com. It explained in depth what an "established" tank was. Maybe some one know what I am talking about and can find hte link.
Some people are like slinkies...they serve no real purpose yet can still bring a smile to your face when you push them down the stairs!:p
Have a great day! Brian