Neutral game mechanics and map components influence snowballing too much
I find one of if not the hardest things to do in league is to take an enemy's outer tower when your both your lane's outer and inner towers are gone without using all 5 team members. This is because you always have to push the wave quite a ways. And once you do, the enemy team has had enough time to react and stop you. Most other helping things (like items and Rift Harold) only help so much when used as the losing side too.
I have noticed that whoever gets first tower (unless it is a trade of towers), often ends up taking the other 2 outer towers fairly soon after. Usually at least 1 using Rift Harold. And then they snowball out of control simply because they now have a global lane push advantage. Losing all outer towers usually means that you are gonna have to wait for baron or some other mid-late game power shift. The wining side will prevent you from pushing out the lane to take towers. Item changes like with how Banner of Command being removed and how Zz'Rott Portal is now terrible in pushing lanes for distance also worsened this. And Yorick's ult is not enough of a guarantee to offset this (since the wining side could have him).
Back when minion mechanics were heavily changed, they made the winning side's minions push the lane better than the losing side's minions could defend it. This was done because RIOT felt that by having minions remain even in strength, it was too hard for the losing team to get consistent farm (because the wave would often push away from their towers or the enemy would freeze the wave). This is/was a serious issue. But the solution was not right. It was more of a band-aid patch. But what should have been a temporary fix, now seem to have just become their solution. IIRC, the solution was to buff the damage of the winning side's minions and buff the defense of the losing side's minions. On paper it may look like a perfect fix, but what it did was it extremely buffed split pushing, and also passive objective taking (winions and such) for the winning side while it made the losing side a slave to their side lanes. Not to mention how it prioritized early tower taking.
Then Riot went and changed towers to combat the passive equal tower trade lane swap style. Again, a good idea on paper. But what it did was make bot lane tower a focus for the early 5 man gank, and a priority on early game superiority for the bot lane carries. Now it is all about who can push faster, deal more damage, and not fall off much later. It is why we see champions like Vladimir, Heimerdinger, Fiddlesticks, Swain, etc... now common in the bot lane. Mages clear waves and/or deal early game damage far better than most Marksmen. So as a result, bot lane towers fall faster.
Then we have Rift Harold. This creature is a snowballing team's BFF. With her on your side, if your team is winning, she will always take 1 tower and may even get 2-3 towers (if properly defended). But if you are losing, she might not even get 1 because you cant defend her against the stronger team. It is also easier for the winning team to secure her as well, since they have the advantage in a fight and do not have to babysit their towers as much.
Next are the Drakes. It is not surprising at all that the winning team often has more drake buffs. Again this is almost directly related to the states of the lanes. If your side is pushed under your inner tower (especially if on the blue side) securing drake buffs is hard work. The common bot tower to drake strategy is still alive and well. But now that many common junglers can for the most part solo or with the help of 1 other champion take the buff, it leaves the rest of the team to prevent the other side from gaining any significant advantages as a trade off. Especially with the cloud drake. Almost nothing else helps a snowballing team keep map control more than the cloud drake's buff, which is one of the easier buffs to solo.
tl;dr:
Basically what I am saying is that the game helps out the winning side even with the neutral stuff more than it does the losing side. There is a significant lack of counter snowball in the game and it seems almost every neutral mechanic seems to end up just inflating the lead the winning side has instead of remaining neutral.