Demand from the West has kept them going there for some time. People in the East were largely unaware of them. Considering that most 'butchers' don't butcher very much (or at all), many didn't/don't know what they are.
Imo, it's been the Net that has increased knowledge of tri-tips outside of the West (this site alone has prompted numerous inquiries, I'd say). I don't know for sure not being a Costco shopper, but it seems that it's quite possible that Costco wasn't carrying them outside the West until Trader Joe's spread east.
Though we have no TJ's in Fla, I've been requesting them from Publix for years, even calling the buyers at their corporate headquarters (who'd never heard of them either). Publix finally started getting them last year, sporadically at my store--and I don't know if they're widely available throughout the chain or not, though I have found them in several stores.
Recently, many if not all TJ's have started carrying tri-tip from Australia instead of the US. The ones I just bought last week in Conn were $4.99/lb, 70-90 cents less than the US tris they used to carry.
Still, as far as East and Mid-west goes, most consumers have yet to discover (and thus demand) tri-tip. Unless they are active foodies on the Net, especially active barbecuers, they do not know of them.
DennisM-- Tri-tips are available in Conn at Trader Joe's:
Danbury*
113 Mill Plain Rd.
Danbury, CT 06811
Trading Hours: 9 am – 9 pm
Phone: 203-739-0098
Darien*
436 Boston Post Rd.
Darien, CT 06820
Trading Hours: 9 am – 9 pm
Phone: 203-656-1414
Fairfield*
2258 Black Rock Turnpike
Fairfield, CT 06825
Trading Hours: 9 am – 9 pm
Phone: 203-330-8301
Orange*
560 Boston Post Road
Orange, CT 06477
Trading Hours: 9 am – 9 pm
Phone: 203-795-5505
West Hartford*
1489 New Britain Ave.
West Hartford, CT 06110
Trading Hours: 9 am – 9 pm
Phone: 860-561-4771
Westport*
400 Post Road East
Westport, CT 06880
Trading Hours: 9 am – 9 pm
Phone: 203-226-8966