If I take you on I want some indication that you're going to stay around long enough for the investment we're going to make in you to be worthwhile. We'll normally pay a recruitment fee and then we'll pay you while you're learning about our product, our industry, our processes and the specific technologies you haven't worked with that we use, and we'll be paying our existing staff to take time out from coding to do those things. That's not inconsiderable.
Obviously your experience may render some of that irrelevant. If that's the case then if you're massively better than the other candidates I may take the risk but if you're not, then I can take you or I can take the person who is as good but will stick around longer. That's not a tough decision.
I'm not looking for gold watch territory or even an indication that you might be around for 5 years, and I'm not looking for a guarantee, but if your CV shows a pattern of you moving every 15 months I need to work out why I think I won't be your next 15 month stint and if I think I think I am whether it'll be worth my while.