Credit card companies use sign-up bonuses in order to entice new customers. Depending on the credit card market, sometimes you will find much more enticing sign-up bonuses than other times. A credit card company may dangle a sign-up bonus out there to convert a bunch of new customers, and then they may pull the offer depending on market fluctuation and other factors.
For example, Chase has re-released their stellar British Airways Visa Signature Card and the huge sign-up bonus that comes along with it: 50,000 Avios points just for spending $1,000 in the first three months that you have the card. If you spend an additional $20,000 throughout the year, that gives you 100,000 Avios points which is enough for two free round trip flights to Europe from the United States.