So, to better understand why in general it is a bad idea to put a support toplane, let's go back to the basics.
First off, why are the lanes divided in top, mid, jungle and bot? Why not everyone in one lane or something else? Experience and gold repartition. There are 4 sources of gold and experience in the map : Toplane, midlane, jungle, botlane.
(also enemy champions, but well they go to the lanes so..)
If you don't use all of these sources of gold and experience, you automatically set your team behind because it's exploiting less resources.
Yet, there are 5 people. Why did we choose to make 2 people go botlane in that case? And why is the metagame built like that (mages/assassins midlane, adc + supp botlane, etc.)?
To understand that, let's go over the main characteristics of each lane:
Toplane : long lane, proximity to rift herald or baron.
Midlane : short lane, center of the map (can roam to both top and bot)
Botlane : long lane, proximity to dragon.
The map is symetrical except for the objectives : rift herald, dragon, baron.
It would seem that the only reason why we put 2 people in botlane is to have more pressure around dragon, which is a more contested objective than rift herald.
Now, the metagame. Why are those 2 people always ADC+support?
reason 1) laning phase : ADC + support in laning is extremely powerful and usually beats or stays even with most matchups.
2) synergies : AD carries have extreme synergy with items, and do not really care about levels. Since they take the CS, they get all the gold in the lane, which means they have similar gold to sololaners, but less experience. Supports on the other hand, have kits that work very well in duolaning and when starving in gold/exp. They don't need that many levels/gold to be useful.
Now, in toplane most champions played there are dependent on levels mostly. They "abuse" base damages to beat their opponent if they're ahead. They care less about gold than other lanes. Because the lane is isolated (mostly because there are more people/interesting objectives earlygame in the bottom side of the map) there is less opportunities to grab a lot of gold. Toplane is the land of minion wave control because grabbing a level advantage is absolutely crucial.
In midlane, champions used have usually one or several of the following features to be a good pick :
-waveclear
-roaming potential
-burst
-range
-instant mobility
In general, they have both good scaling with items AND gold. That's why they get the lane in the center of the map, where the minions arrive first so they get exp/gold slightly more quickly; and to fight in a short lane, they need either range or extremely high mobility. We see mostly burst people in the midlane also because it is a short lane, you can't chase for long before running in the enemy tower. So in general bruisers/tanks are somewhat weak in that lane.
With all those preliminary explanations, let's now say why you shouldn't bring a support toplane :
- You're setting your laner automatically behind in experience. If the enemy laner is very good and knows how to just stay in minion exp range and doesn't even take CS, he's still ahead in terms of exp and might just beat down your laner as soon as you leave the lane.
- You're not putting any pressure towards dragon. The enemy team is more likely to grab early drakes.
- Your ADC botlane is likely getting more experience, which he doesn't care a lot about, but he's losing on creeps since if the other botlane isn't retarded ; they'll just deny him. Also, ADCs aren't very good at surviving towerdives because they don't have the kit for that and don't build def items early on.
(Alternatively, you can put someone else botlane. But except if you take an ADC top to compensate, your team is going to lack a reliable range DPS carry. In any case, if you just get ADC+supp top and sololaner bot, you lose out on pressure around drake. Herald doesn't spawn before at least 10 minutes, and at this stage of the game in high elo games, it's like there are no real lanes attributed anymore ; unlike low elo where lanes last forever).
Hope I've helped.