read last months' "The Source." Good article about Dame & Jay and how shit started. Also, do some research before you ask questions.
Why was Rocafella/Rocawear successful? Because Jigga made them big and they kept up w/demand. You think if Jay was a flop that any of their shit woulda sold? Hardly. They occupied a niche and eventually flooded the market.
I'm not big on Rocawear's history, but here's what I know about Rocafella:
Dame went to high school w/Cameron Giles, Mason Betha, and Lamont Coleman (Killa Cam, Murda Mase, Big L). Dame was always that dude that didn't have the skills himself, but wanted to be "the manager." Even in high school, Dame was Mase's pretend "manager." (this is all in Mase's book Revelations.) So, yes, this means that Dame was from Harlem.
For some reason, Dame ended up being Sean Carter's right-hand man. The odd part about this is that Sean Carter (Jay-Z) is from Brooklyn, whereas Dame is f/Harlem. There were multiple rap groups/camps forming all over NYC at the time (early 90s), w/a few of these being Ruff Ryders (Dee, Waah, DMX, Dame Grease, Swizzy, The Warlox -- later known as "The Lox"), Rocafella (Dame, Jigga, a few lesser known cats), Murder INC/CMC (Ja, the Gotti Brothers, O-1, Chris Black), Children of the Corn (Killa Cam, Murda Mase, Bloodshed).
Most of the groups came in contact w/each other during battles at local rec centers or on the streets. In DMX's The Autobiography of D.M.X., X talks about battling Jay-Z in the early 90s. Apparently, both X and Jigga were the most skilled cats f/their respective camps, and ended up having to settle the battle that was started by the "Pups" (what X called Jada and Styles) and Jigga's guys. X claims that, after about an hour of battling, Dame actually admitted that "X got [Jay]."
One thing that most people don't know is that Irv Gotti is the guy that kicked everything off for so many New York rappers. X's boys Dee and Waah (hustlers who founded/funded Ruff Ryders) actually bought Gotti his first sampler, and Gotti really does have talent making beats. Gotti became really close w/Ja Rule and soon became an A&R @ Def Jam. Def Jam gave him a 50/50 deal to start "Murder INC," and Gotti's next move was to help Dame put together Rocafella underneath the Def Jam umbrella. Lastly, Gotti helped orchestrate the Ruff Ryders imprint underneath Def Jam. He did this by bringing Lyor Cohen (CEO of Def Jam @ the time) to the original Ruff Ryders' Powerhouse Studios so that Lyor could watch DMX rap.
To start Rocafella, Dame and crew shot a low-budget video for some Jay-Z song, put out some street mixtapes, and created some buzz. Once they got signed to Def Jam, they milked it for all it was worth. Read the article in the Source.