That's an interesting observation that you make.
My Illogical Theory
Dragon has always been considered a generally OP type due to their lack of weaknesses and how good Dragon-type Pokemon tend to be stat-wise. The superiority of Dragon-types seems justified though. The other types represent generic elements like water, electricity, ice - you see them all the time and they are generally harmless (in most cases, anyway). But dragons!? THEY'RE SO COOL. Unlike the other types, Dragon-types are associated with mythical, fire-breathing creatures that could destroy anything and everything in their path. Even debatably mythical creatures like Fairy- and Ghost-types are a hell of a lot less frightening than Dragon-types.
And I think that's why Dragon-types are resistant to the Fire-, Water- and Grass-types. These types are the first types you encounter - they're the types that you would be most familiar with, the complete opposite of the mythical fire-breathing dragons. I imagine Electric-types fit this criteria as well, but I couldn't be too sure. Among all the non-starter types, I would think the Electric-type is the most 'familiar' type.
Of course, we can't know for sure because only Game Freak knows blablabla. It's an interesting observation you make, though!
Hope I helped. :)