It was a combination of things. The initial car accident knocked Agent Grogan's glasses off and caused 2 other agents to lose their revolvers. Platt was hit 12 times, but wasn't stopped until a .357 hit him at nearly point blank range. The other agent who was killed, Dove, had his pistol disabled after it was hit by a round from Platt. Unfortunately, Platt was very lucky that day. If the scenario played out 99 more times, it couldn't possibly have ended as badly for the agents as it did. Matix was basically out of the fight from the beginning. Platt did all the damage. Here is a breakdown from Wiki on how many rounds were fired and by whom.
http://en.wikipedia.org/wiki/1986_FBI_Miami_shootout
Agents
Richard Manauzzi: lost control of weapon in the initial vehicle collision, no shots fired. Minor injuries from shotgun pellets.[24]
Gordon McNeill: S&W M19-3 .357 Magnum revolver, 6 rounds .38 Special +P fired. Seriously injured by .223 gunshot wounds to the right hand and neck
Edmundo Mireles: Remington M870 12 gauge shotgun, 5 rounds 00 buckshot fired, .357 Magnum revolver (S&W, model unknown), 6 rounds .38 Special +P fired. Seriously injured by a .223 gunshot wound to the left forearm.
Gilbert Orrantia: S&W (model unknown) .357 Magnum revolver, 12 rounds .38 Special +P fired. Injured by shrapnel and debris produced by a .223 bullet near miss.
John Hanlon: S&W Model 36 .38 Special revolver, 2-inch barrel (5 rounds .38 Special +P fired). Seriously injured by .223 gunshot wounds to the right hand and groin.
Benjamin Grogan: S&W M459 9mm pistol, 9 rounds fired. Killed by a .223 gunshot wound to the chest.
Jerry Dove: S&W M459 9mm pistol, 20 rounds fired. Killed by two .223 gunshot wounds to the head.
Ronald Risner: S&W M459 9mm pistol, 13-14 rounds fired, S&W (model unknown) .38 Special revolver, 1 round .38 Special +P fired. Uninjured.
[edit] Suspects
William Matix: S&W M3000 12 gauge shotgun, 1 round #6 shot fired. Killed after being shot six times.
Michael Platt: Ruger Mini-14 .223 Remington carbine, at least 42 rounds fired, S&W M586 .357 Magnum revolver, 3 rounds fired, Dan Wesson .357 Magnum revolver, 3 rounds fired. Killed after being shot 12 times.