This is not the reason but a key reason: Baltic rough sea ship building tech met Mediterranean large scale ship building tech via the vikings. Outcome is caravel hulls. Strap cannons on and now your ship is the naval equivalent of a nomadic horse archer army. You can hit anything with cannons and just retreat to blue waters before anyone could hit back.

Mediterranean ship building was built for scale but mostly joined large blocks of light wood together using mortoise and tenons. This meant ships could be large but were also brittle. Rough seas would snap the boat. But there were still large enough that people could put cannons on them as soon as small cannons were a thing.

In the Baltic, the seas are always rough. So shipbuilders used thin overlapping strips of heavy wood, which were nailed together, called "Clinker built". These thin strips of wood could twist and flex in rough seas so your ship wouldn't break. But you could only build ships so big in this style. Viking long ships could never fit a cannon.

Were viking traders first met Mediterranean shipping would've been around the Iberian peninsula. So its little wonder that the Spanish and Portuguese were the first intercontinental European empires. The Portuguese were considered the "first" though. And were far more Naval based than the Spanish. Once they got into the Indian ocean they became sea Mongols because they could strike anyone with impunity, claiming dominion over the entire indian ocean. Check out Afonso De Albuquerque for Portuguese exploits in the Indian ocean.