An insight on why it feels like bot lane decides games

Top lane champions, mostly tanks/bruisers rely heavily on their base damages and base stats. So even if you're ahead as a bruiser, you're not THAT much more effective than if you were behind, and vice versa (being behind doesn't make you that much less effective than behind ahead). So even if you win your lane hard as let's say Jax vs Ornn, or even Jax vs Darius - the Darius/Ornn still provide a lot in teamfights.

This gets to be a little less prevalent in mid lane, but still true. Mages rely HEAVILY on their base damages, AS WELL AS the fact that they usually provide some really strong CC. So even if your let's say Taliyah is dominating the enemy Orianna, the enemy Orianna will still have a lot of damage/utility. Or vice/versa. Taliyah still brings strong zone control, and plenty of base damage. Also the reason you sometimes see mages in support - they don't need income to function (however its not that common because they do need levels which they can't get as support).

However the problem when it comes to ADCs is that they rely HEAVILY on items and their base damages are almost negligible as the game goes on. That means if the enemy Vayne dominates your Tristana... guess what? Vayne now has IE+shiv while Trist is sitting on IE + dagger - and since both their PRIMARY function is to do damage, Vayne is SO MUCH more useful than Tristana which can heavily swing a fight. On top of this their items scale multiplicatively - so Vayne behind ahead starts to REALLY feel bad.

Since both players scale with gold, one player being ahead and one player being behind makes that gap even more noticable (i.e. if you ADC was Ashe instead, then she'd still be more useful in this situation)

So what does this mean

A lot of this has to do with the meta. What do I mean? If you have Yasuo vs Fiora top, or Riven vs GP, it will feel like top lane snowballs decides games more. Since these champions rely heavily on gold, and they scale well with gold - them snowballing is going to be very effective (ESPECIALLY since the enemy also scales with gold, so them being behind also really hurts their effectiveness).

Or if mid lane was Akali vs Zed, or Kat vs Talon. Same principle applies here.

Or if crit itemization got nerfed so ADCs snowball less hard, you'd see more Ashes and Varus (not because he abuses rageblade, but for his CC).

TL;DR: The current meta makes it favorable to pick non-snowbally* mid/top laners which makes carrying through them difficult. This might be because ADCs snowball harder than any other class - therefore its easier to play around them snowballing than any other lane; however that doesn't mean top/mid can't decide games via snowballing - it just depends on the matchups.

*If the baseline effectiveness of both champs in the lane is high, then they do not snowball hard. ADC baseline effectiveness is low and they need items - this is why they will snowball very hard