I would say its a combination of the two.

I would say its a combination of the two that gives the real problem. You stated that 3,/4/5 cost cards can be drawn. Yes they can but out of 30 cards, say you run 2 doom, 2 harvest golems and maybe shattered sun cleric or void terror depending on what you actually run with the zoo deck and have in your collection. That is a small amount of chance to draw those cards all together. And while we are on the subject of chance, SC2 isn't so much about chance as it is about skill. There is no RNGenerator that says I will gain a topdeck advantage against a pro. That pro will win every time no matter what. I don't care how many times I rush an all in or some play I've "mastered" against a pro SC2 player, I simply will lose. Back to Hearthstone, I have a good chance to win against Kolento or Trump even though they are considered the "Pro" players. There simply isn't enough skill involved in hearthstone besides knowing the cards, what is run in the decks and how they may be used. Mechanics in hearthstone are not the seperating factor in determining skill between a player amongst other things like in SC2. Look at MTG, after 20+ years they mostly have been running copies of older cards in new graphic formats, very slowly putting in new mechanics. The skill required for that game is higher yes but yet again, it is only knowing the decks, and how you would respond. Beyond that, its hoping you draw the right cards. So back to drawing, so if I gain the ability to draw more of my deck and most of my deck synergizes then I have a higher chance of winning than other decks which require time to combo or put out my big plays. As of right now I am 5 on the ladder and all I see is zoolock/priest and a smattering of people trying to break through to legend by playing decks like druid etc that usually can innervate or power their way through to a faster recovery versus zoo and priest decks.