How much does Jayce rely on auto-attacks (which benefit from crit) vs how much does he rely on purely AD-scaling abilities - Q (cannon) Q (hammer) and E (hammer) - (which get no benefit from crit)? Given that, how much of a benefit are you really getting by buying a more expensive IE instead of a BT? And then how much are you delaying other items in your build by buying IE instead of BT? It's a question of gold spent vs benefit gained.
Just because I haven't had much time to play beyond a quick co-op first win of the day for the past few weeks doesn't mean I can't do simple math.
The basic theory is that maxed hypercharge acts as an attack resetter, AND has a 3.9 AD ratio (1.3 AD autoattacksx3).
So thats a 4.9 ratio that happens in about 1 second.
Copying a piece of it for illustration: (note I don't assume the attack reset for hypercharge, just a 3.9 ratio)
Assume 300 AD, no crit.
Q/E Nuke does 400 +200x1.68 = 750ish damage.
HyperCharge does 3.9x300 = 1150ish damage.
Q melee gapclsoer does 200+200 = 400 damage.
E hammer knockback does 160+200+20% = 750ish damage.
Imagine we can do 1 of 2 things for sake of comparison: Get 90AD or 75% crit (I am assuming the Inf Edge passive in effect which you obviously can't have with 0% crit but go with it for now). Both have similar costs (40g per AD, 45-50ish per Crit)
HyperCharge would do 3.9x390 = 1500ish damage with the +90 AD approach (up from 1150). Other skills would get about +90 dmg (or 140ish Q/E nuke)
With the crit approach, we are looking at 3.9 x 300 total AD +75% chance of adding 160% of 300 AD damage.
== 3.9 x (300 + .75x1.6x300) == 3.9 x 660 = 2574! (up from 1150)
Wait...whaaat? That's right. The crazy 260% multipler from crit turns HyperCharge into an amazing engine of DEATH, once a decent amount of AD is obtained.
Specifically, for the same gold-cost that crit increases HyperCharge damage by 120%, compared to the 25% or so gain from more AD (though the AD also boosts his other skills too).
THIS is why we focus on crit lategame because it increases Jayce's burst potential to ridiculous levels. Basically we don't try to turn him from a burst caster into a sustained output ADC, but instead tweak itemization to use HyperCharge to increase his burst exponentially during the endgame.