I think i big part of it was that everyone was talking about Minecraft, to a much large extent than they are about Terraria, and a large part of that conversation took place in the form of or in relation to YouTube videos. The videos were impressive and made people want to share them, and that in turn made people want to try out the game.
If you look around on YouTube for videos about Minecraft you can find a couple distinct kinds:
1: I built (or found) something huge and impressive.
2: I built (or found) something beautiful.
3: I built something geeky.
4: I was overwhelmed by a horde of monsters and killed.
5: I was overwhelmed by a fire or lava and killed (or had my stuff destroyed.)
6: I was surprised and blown up by a Creeper (or had my stuff destroyed.)
The first three three are often found together in some combination. A huge and impressive model of the Enterprise, a giant working CPU using redstone circuits. Recreating large swaths of Middle Earth. They can probably all be summed up as the "Awesome" category.
The last three don't happen together in combination as much (though they can certainly all happen sequentially in longer videos or series of videos like the Yogscast) but they can likewise by summed up as the "Lolz" category.
So why is it that Terraria can't compete?
First, Terraria is not good at the Awesome. You can build large structures, but because it's a 2D game you're limited in both the scale and perspective. Your view is always centered on your character and you're always looking at a 2D interior slice of what's around you. There's no way to back off and get perspective on the entire thing from an external viewpoint, in relationship to the landscape around it.
This is kind of a weird thing for me to say because i _love_ 2D games and have trouble playing a lot of 3D games. I enjoyed Link to the Past and Super Metroid vastly more than i did Ocarina of Time and Metroid Prime. However even i have to admit that the kind of vistas you can show in a 3D setting are much more impressive visually, even if i personally don't enjoy playing in them as much.
People _can_ spend hours working on projects in Terraria, and people familiar with the game will recognize how much work went into those projects, but you rarely get the same visceral sense of awe upon seeing the results.
Second, Terraria is not good at the Lolz. Despite my incident with dynamite the other day, crazy shit just does not happen anywhere near as much in Terraria as it does in Minecraft. The challenge level is much higher in Terraria; enemies are always spawning and you're always fighting them so there's very rarely any surprise in encountering an enemy, and getting killed by one is usually pretty insignificant as far as the entertainment value goes. And part of that goes back to the 2D/3D thing. In Terraria darkness can be a hindrance, but for the most part you can see 360 degrees around you for a fair distance. It's hard for something to really sneak up on you.
There's also not many ways to do damage to the landscape. You can blow stuff up with dynamite, but that doesn't actually _destroy_ anything, and you certainly can't set stuff on fire.
All the ways that Minecraft is different in this regard are epitomized the the Creeper. Creepers, like all monsters in Minecraft, do not spawn especially frequently, so you don't expect to run into one as a matter of course. You can't see them coming unless they're in your line of sight, and they don't make much noise, and of course if they catch you they explode violently, probably killing you and certainly damaging or destroying a lot of stuff in the vicinity.
You can be wandering around working on some project or another, then turn a corner or break through a wall and suddenly find yourself face to face with a Creeper. You will probably scream or swear, frantically try to get away, and quite possibly get exploded. If you happen to be recording at the time it frequently results in a hilarious YouTube video of the schadenfreude variety.
Of course the humorous possibilities aren't limited just to Creepers. You can get ambushed by packs of other kinds of monsters, you can get shoved off a cliff by a sheep, you can accidentally set your house on fire, you can accidentally dig through a wall or floor and unleash (or fall into) a flood of lava.
Terraria _is_ good at the Fun. But when it comes to massive success as a meme that's just not as good as the Awesome or the Lolz. I don't feel compelled to share most of what happens or is created in the game except in the sense that i will share details about any game i'm playing with friends who are also playing the same game. Nor do i feel compelled to go looking for cool Terraria videos just for fun. If i look for Terraria on YouTube it's because i'm looking for advice/suggestions on how to handle a particular problem.
In fact i think it's the fact that Terraria is in 2D, combined with the focus on gameplay, that makes Terraria more fun (for me anyways,) but it's also what makes Terraria less interesting for people to talk about than Minecraft.