naive

"Why is airfare to Stockholm significantly lower than other European cities?"

There is no longer much logic to airfares. When I was looking several years ago, Stockholm actually cost more than most other places. So, if you found a good fare for a route that meets your needs, use it.

"Is this a good way to enter the continent if planning to visit Scandanavia, Denmark, Czech?"

Yes, yes, no. It's obviously good for Sweden, and connections between Sweden and other Scandinavian and Nordic countries are good. But the Czech Republic is not close to these places, so you're looking at a flight or a VERY long train ride with several changes.

When I flew SAS from Newark to Scandinavia (into Helsinki and out of Bergen), they had a deal where you could add flights within Scandinavia cheaply to your ticket. Check if they still have such a plan. For other connections, you can look at rail schedules on Bahn (German Rail) using Rick's tutorial: https://www.ricksteves.com/travel-tips/transportation/trains/online-schedules. You can find flights within Europe on Skyscanner: http://www.skyscanner.com/

Thank you, useful information

SAS tends to be on the less expensive side and you can easily catch flights from Stockholm to other parts of Europe. And depending on where you live in the US/Canada, it can be a quicker flight to Sweden than to other parts of continental Europe. It actually can make sense to connect there to parts of eastern Europe.

Thank you very much - that is encouraging too

One other reason for the airfare decrease to Stockholm is that Norwegian now offers service to Stockholm from JFK. They are a discount carrier that has significant presence in Scandinavia...they now do a transatlantic run. Oddly, the are called Norwegian but they have a huge hub at Stockholm's Arlanda Airport. I have flown them once from Helsinki to Stockholm and enjoyed the service.

Todd's explanation solves the "mystery." When a new airline starts flying a route, they often discount it to attract business. The other airlines flying that route often match that discount. Unfortunately, these lower fares are "introductory" prices. Once the introductions are over, the prices jump back up.

So, if you're interested in that route at that price, book ASAP.

Same odd pricing situation for us last summer as noted by the original poster.