It's a reaction to the evolution of society. For a long time "churners" were a very very small minority. Then it grew. With the proliferation of people learning about how to work the systems and then blogging & "travel hacking" blah blah blah the companies are going to protect their bottom line.

Also, don't forget that the banking system has changed over the past 5,10 & 20 years. Laws have changes that make it harder for banks to give away bonuses and make a profit while also getting a valued out of the customer in the medium to long term.