I take it you are not looking for the botanical reason orchids are different from other plants, but the reason why they have us all hooked?
I think for me there are multiple reasons...the beauty, for sure. The challenge to grow and flower them well. I find it fascinating that there are so many different kinds, with so many different and unusual variations. I could learn something every day about orchids and never learn it all. I guess learning about them is really another type of challenge.
And, they also have a fascinating history.
If you were to equate flowers with fabric, orchids would be a very dense, rich tapestry, with layers and details hidden in so you were always seeing something new in it.