The thing about underdog is this.
Our culture as produced in books/film/tv/music is dominated by left leaning beliefs.
If your conservate the media that naturally surrounds you does not reflect or celebrate your values.
Though I'm not saying they don't exist.
If you look at a demographic map of blue vs red, you will notice that blue = cities, red = rural. Even many blue states are only blue becuase of their cities.
This can create a feeling of "underdog" in both directions. Blue may feel like islands in a sea of red, red may see as wealth and power bring concentrated in blue lands.
The academic establishment is left leaning. That is a big one you leave out.
Finally regarding the lack of prominent left wing internet communities, this.
I live in the bay. If I want to have a honest discussion about say, basic income I can just step outside.
The same is not true of say, opposition to minimum wage. That will only get vitriol thrown at me and damage my personal reputation.
The left has few prominent internet communities because they don't need them. The right leaning people do, they are either very spread out in real life, or exist in communities that don't agree with them.
The same is true online. I don't need a left wing forum to discuss left wing ideas because every internet community I'm part of is predominantly left wing.
Basically if there is an internet community that's not expressily rightwing, it's community will be leftwing, so leftwing people don't need a space for themselves.
The lack of a prominent left wing forum is simply because the left is so dominant they don't need it.
Finally I we would note that "elite need the common man" is an accusation leveled at both parties.
For example my mother who is really right wing that most politicians who are pro immigration aren't interested in the welfare of immigrants, but just want more people to vote for them.
This lack of sincerity the motives both sides perscribe to each.