I'm not sure if I understand your question correctly, but I'll try and see if I hit it somewhere near...
1.) If you're trying to say why they didn't reclaim land and build additional runways, and expand the airport. That's because of the geological location of the airport itself. When you look at where Kai Tak is on a map, you'll see that's it's surrounded by hills on the north side, east side, and hills on the HK island in the south direction. Where the IGS 13 beam is, is the gap bearing a westerly direction.
However, due to the Meteological(sp?) conditions, 80+% of the time, winds favour landing and departing on runway 13. However, If we look at building another runway, say, parallel but to the west of rwy 13, hills on the HK island will block the departing and arriving traffic, and if they are using rwy 13's departure path, then the second rwy hasn't solved anything.
If the second rwy isn't parallel to rwy 13, it will take up a whole lot of space, and we all know what the enviromentists will say...
Therefore, there wasn't many choices, but to build a brand new airport, somewhere that they can plan expandion, and is not too near the main parts of the city. And I think Chep Lap Kok island is a good choice. 1, it's close, but not too close to anything, or everything. 2, it's kinda sheltered from the ocean, so even under heavy storm, atleast final approach will be kinda stable.
2.)If you mean by why they aren't using it for somthing else now, there are a couple reasons. First, oil and fuel used when the airport was in operation was left behind, leaving the land toxic, and unless someone wants the land very badly and is willing to pay to clean it up, it'll stay like that for a while.
Second, HK's current economy looks more like a piece of burnt toast, than anything. Low-cost housing is in need. But land Kai Tak is on, is prime, high price-range realestate, so unless it's the HK government that's building the houses there, no thanks.
So therefore, if nobody has something to do with it, just leave it the way it is.
Please feel free to correct any facts that I'm wrong, and most of this post is my personal opinion, so, if you think I'm wrong, feel free to say your opinion.