It's built into the design of bitcoin. It automatically adjusts the difficult of mining up (never down) based on the rate at which coins are mined in heats. Every time a faster ASIC for mining comes out, the difficulty shoots up correspondingly. At best you can mine a good percentage of the blocks in the current heat with a new machine before the difficulty shoots back up and the new ASIC performance is the new baseline The new asics are still better than the poor schmoes running regular cpus or gpus, but it's quite difficult to "get ahead" with mining unless your mining resources are free, such as harvesting cpu cycles form a botnet.. There are a finite number of bitcoins, and with every block of them found the difficulty to mine the next batch goes up very fast.
I recently thought about getting some ASIC mining hardware, but after running the numbers and factoring the cost of electricity, and the current price of bitcoin, it was dubious whether the devices would even make back their cost even if they did ship on time and perform as advertised. Even just running mining software on cpus and gpus I already own is a losing proposition due to the cost of electricity. Not really a good investment, unless you're on unmetered power, such as college dorms.
The way that that the mining difficulty cranks up with blocks returned, the developers of any new fast ASIC hardware will reap the greatest benefit of the faster hardware during development, and by the time you get it in your hands the bitcoin ecosystem will have already cranked up the difficulty. It will still be faster than older hardware, but since the difficulty cranked up too, it's likely going to produce much less than you initially expected.