There is no "East Coast regional sport" as you like to call it. Sure, ESPN covers East Coast teams a little bit more, but the majority of the country resides on the East Coast. What can you do? In terms of actual attendance numbers....
MLB actually had a great year for attendance in 2013. Ironically, two East Coast teams ranked near the bottom for attendance. Even more ironic is that both teams made the playoffs (Cleveland and Tampa). The Mets were ranked 21st in the league. The Marlins didn't sell out a single game. The Phillies saw a 16% attendance drop from 2012. Pittsburgh had their first winning season since 1992 and made the playoffs and still ranked just ahead of the Padres for attendance (#20, which is still respectable).
The Giants and Dodgers are perfect examples of teams on the West Coast with strong attendance numbers. The Dodgers actually led the league in attendance last year. The Padres are middle of the pack (#21) but they're terrible. They still sell out on most weekends though. Seattle is near the bottom (#26), but they still posted higher attendance records than 2012.
If you sort by attendance, the first 5 teams are:
1. Los Angeles Dodgers
2. St. Louis Cardinals
3. San Francisco Giants
4. New York Yankees
5. Texas Rangers
Notice how only one of these teams is an East Coast team.
The bottom 5:
26. Kansas City Royals
27. Houston Astros
28. Miami Marlins
29. Cleveland Indians
30. Tampa Bay Rays
Notice how (the bottom) three teams are East Coast teams. Tampa made it to the freaking AL Divisional Series for crying out loud.
There is no "East Coast regional sport" aspect at play here. As a whole, MLB posted their 6th highest attendance numbers of all time, and as you can see, East Coast teams, Midwest teams and West Coast teams are scattered all over the place for attendance numbers.
While not entirely at the bottom, Oakland comes in at #23 in attendance and that kind of surprises me. Oakland is just a terrible market. My dad and I went to an A's-Indians game last season in Oakland. Both of these teams were teams destined for the playoffs. The A's finished first in their division last year. We went on a Friday night. There was only about 10,000 people in attendance. It was terrible. The stadium is falling apart, it's surrounded by barbed wire fences and scrapyards, literally the entire upper deck is covered in tarp to help avoid Raiders blackouts, and they sure as hell aren't seating anybody up there for a baseball game.
I'd love to see baseball in Portland.....but I'd like the NHL more. Portland loves their sports teams, and I'm sure either one (or both) would do well.