Why didnt he cast Defile? Sylvanas would have been taking a huge ticking dot and never would have been able to channel her spell.
Because he was not geared up and haven’t been doing his mythics 15+
because he has been afking, he was watching some cute cat videos on youtube
Because he is the Lick King, not the Lich King. Easy mistake to make.
Because WoWs only undead uber pally forgot to bubble hearth
Wouldnt his hearth have been ICC though?
He had the Remorseless Winter talent, didn’t take Defile. That’s the last time he listens to Noxxic for talent advice.
Sylvanas had plot armor.
Bolvar was a powerful paladin and he never was a proper death knight – in fact I’m not even sure that he’s actually undead – he’s just a dude who can’t die because of red dragon fire.
The Helm of Dominance affords him some level of death-knighty powers but that only goes so far, and it’s not like he’s been roaming around Northrend practicing his skills.
Because he made Pandaren Death Knights and Sylavanas wanted to let him know the joke had to stop.
Our God-Queen is all powerful. She can never be defeated. 
Short answer: plot.
Long answer: Bolvar did not have access to Frostmourne, which was one of the key reasons Arthas was so freakishly powerful. Additionally, Bolvar’s goal was simply to keep the Scourge in check; he wasn’t concerned with power like Arthas was. Sylvanas was backed up by the Jailer’s power, so it’s not unbelievable that she could beat a “weak” Lich King like Bolvar.
Bolvar was a boosted lich king. No idea how to play his class.
Sylvanas waited until he’d blown all his cool downs and then CC-locked him long enough to steal his only Legendary Item and destroy it.
GG
You’re all wrong. She simply kited a level 120 trash mob to ICC, used misdirect and popped feign death.
Sylvanas has a new sugar daddy that’s feeding her the power of the tens of thousands of souls she had killed since the beginning of Cata. Bolvar could’ve had Ilidan and Arthas backing him up and it wouldn’t have mattered because of how obscenely powerful they’ve written her to be.
Werent you watching? He lost because he changed specs from blood to frost before he fought her!