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Some of it was just the general fashion style and attitude of the early 1990's with neon colors and bright, fun stuff being trendy. Hand drawn logos also used to be a lot more common (and are superior to digitally drawn logos).
There's also just not a lot of characters anymore. It's not hard to come up with shirt ideas of Jake and Brutus Beefcake. Like hey, lets just throw a couple of snakes or pairs of scissors on there. It's a lot harder to come up with something fun for people like Roman, Omos and Mustafa Ali.