If you have deflation together with a large debt bubble (like we have now), many people & companies will have to default. Not saying it's good or bad, but politically that kind of pain is intolerable (the politicians who support it would get voted out)

If that leads to your income being reduced (or you're downsized), you have less to spend and your contribution to demand goes down. That starts the process all over again with whatever you consume. The worst case is a "deflationary spiral" where that keeps going round and round.

It's widely thought that a small (2%) inflation avoids that spiral by encouraging people to spend their money today rather than sit on it. It slowly eats into fixed incomes, which isn't great, but in general it keeps the economy moving for those who are employed. Ideally it pumps the GDP faster than inflation, allowing us to compensate for those on fixed incomes.