Why are all popular crypto currencies deflationary?
There's no technical problem with making an inflationary crypto-currency. For Bitcoin, just changing the block reward schedule would do it. And there do exist crypto-currencies that have perpetual block rewards.

The thing is, the set of people who believe that inflationary currencies are good doesn't significantly overlap the set of people who think that currencies don't need to be managed by central banks. So there's not much of a market for such currencies. It's kind of like a veggie burger with bacon.