From what I've heard Lisbon is about the same cost as Berlin. But the economy is worse, the tax system seems more complex and it's less accessible to foreigners. I guess the weather is nicer, but the location is worse. To legitimately compete with Berlin I think one really has to offer something special, otherwise Berlin wins by default in the low-medium cost category. Lisbon would still be on my shortlist for European bootstrap cities, but there are also at least half a dozen other cities that could be equivalent.
For people to choose something it first have to be a realistic option. Berlin is much more established as an international and "tech" city. You hear about people moving there all the time. For whatever problem you'll face there's bound to be someone else who has done it. When it comes to moving you don't want to be a pioneer and spend thousands of euros finding out if it's doable and ending up not liking it. Being in the same boat as other people helps with that uncertainty.