let me explain this, because maybe I put too many insults into my last post. That was my problem sorry.
here's how I view things:
Dragon is not like old dragon. Old dragon was equally worth when you were behind, as it was when you were ahead, since it gives flat gold. Indeed, since it gives flat gold, it is more important when behind, since giving flat gold decreases the percentage that the enemy team is up against your team.
Now, Dragon is a percent boost. It is worth less for teams that are losing, than it is for teams that are winning, since it boosts a percent of your damage. For teams that are losing, you want flat gold, so you can buy items. A 6% boost when their ADC is sitting on Pickaxe and 500 gold is not that big compared to if you took a tower and now your ADC has a BF sword, support has another cloth armor, closer to Mikhael's, etc. Remember that if your mid is behind, contesting dragon is not even a sure thing because by the time your mid is behind far enough (even 0-2 is substantial) then a teamfight at drag is a gamble.
Meanwhile, when their mid is at dragon, your mid takes 10 extra CS, your bot lane takes 10 extra CS, you kill enemy top laner and get their tower - you traded a 6% AD/AP buff (worth approximately 300 gold midgame to top/mid/jg/adc, not much worth for support, so 1200 gold team advantage) for 300 minion gold top/mid + 625 global turret gold + 300 kill gold + 150 assist gold -> this is 1375 team gold, approximatly even, with the difference now that top is threatened, your mid is catching up on EXP and their top laner is missing exp.
This trading of objectives is far more worth than trying to contest dragon, getting 3 for 1 or 2'ed, and maybe even losing dragon anyways, beacuse your mid was behind.
Mid is a hard lane to gank for especially early game. Top and bot are the easiest. That is because mid is the shortest lane, there's less room to maneuver, counteganks can happen from anywhere and its hard to dive too far since there's 2 turrets in a row at mid. Additionally, most mids have substantial range and CC, meaning that dives are extremely risky. It is better to force their mid to leave the safety of their lane and help top/bot, where they can be ambushed in river. If their mid is Zed or Fizz, it is a total waste of time to try to gank, and then you'll end up with a weak jungler and a weak mid. It doesn't matter if Katarina on paper carries harder than an Udyr, because Udyr with gold can 1v1 almost anyone, take towers in seconds and force 1v3's and still run away alive, while ganking for say Katarina might mean that Udyr is spending 30 seconds to a minute not farming, not ganking other lanes, not doing anything, other than waiting for a chance to go in, while Kat has zero CC and low damage since she's behind. Same with J4; a fed J4 that builds damage can take towers in seconds and escape anything more than a 1v1, but if he camps mid, he might be spending tons of time there that he's not farming, not ganking elsewhere and not taking towers. That means both mid and jg are weak, and while a 4v5 is winnable, a 3v5 isn't. Even more true for Yi, Warwick, Jax, etc.