I think part of the "Scrub Friendly" tag that has been appended comes from the easy inputs in the game in general, which makes the game more accessible from the beginning. Combos/juggles are typically not as hard or long, strings are easy to input, buffers relatively friendly. What's more the stuff that IS hard usually only gives you a marginal leg up on opponents (maybe a little more damage or more RO distance on a combo or maybe a slightly faster/safer poke or combo starter than other options), so manual dexterity is not rewarded very much. Players of Tekken, Guilty Gear or Street Fighter, for example, will be used to tough inputs they had to put a lot of time into to get down which went a long way to make them better than their opponents. When that same player looks at SC and sees someone getting lots of mileage out of some simple bread-and-butter poke like a BB/AA and throwing frequently of course they'll think it's scrubby, how much did he have to practice to learn to do that?
There is some extent to which they are right too, playing a well balanced fighter with a high execution requirement IS harder than playing SC. But very few people are really playing those games "as they was meant to be played"(and to those who are I salute them). Where SC shines is for all those players who care about the strategy of the game(learning move uses, match-ups, character vs situation options, positioning, reading an opponent, etc.), they don't have to practice their inputs for forever and a day before applying what they've learned can start winning them matches.