Bitcoin can inflate. It's just that in order to change the current agreement on what Bitcoin is and how it works, you need a majority of mining power to agree, but the Bitcoin community has been famously conservative and unwilling to any kind of change. The reason? Probably a small amount of fear, and a huge investment in ASICs that would not be able to work if Bitcoin changes.
Also the Bitcoin community is, almost by definition, made up of people holding a lot of Bitcoin. They have a strong financial incentive to make sure that it deflates instead of inflates so that each Bitcoin becomes worth more over time instead of less.
