People have hoarded them, thinking that they will be valuable. The US Mint is still minting them, and have minted 6,160,000 Sacagawea Dollars through September. They have cut the number they are minting. In 2001 (the first year of the coin) they minted 133,407,500 of them. In 2002, they minted 7,597,610 of them. The only way that the Golden Dollar would take off would be to phase out the paper dollar; unfortunately, that will never happen.
At the amusement park I worked at over the summer, they gave them out all the time. I hate having to carry change around, much rather have a george washington in my wallet instead. When I went to canada their smallest bill was a 5, it was ok I guess because coins could be used everywhere no like down here.
I remember Wal-Mart having them, but I've only gotten them from the stamp vending machine at the post office. I believe that they are not being used very much outside of vending machines is that they are just inconvient to carry when compared to paper bills. Plus, the girls at the strip club don't like it when you toss them hard metal coins instead of soft paper bills.
We have had a similar thing here in the UK with people hoarding them. The mint has produced coins for special occasions. They are legal tender but people save them ( and they will never increase value either). Result is the Treasury makes a fortune because they know that they will never be cashed in. If it was an individual doing this it would be labelled a scam but because its the government its okay.