Just found this short story by Black Library author supremo Aaron Dembski Bowden which answers the question "Why does Chaos no longer have any Whirlwinds or Land Speeders when they had them during the heresy?"
"Dude, Horus just croaked. What now?"
Abaddon re-tied his topknot. He totally had an idea. "I totally have an idea," he said.
The Traitor Legions looked on expectantly.
"Here's what we do. We run away."
Many helmed heads nodded. This seemed a wise course of action.
"Good idea," said Erebus.
'Hush." Abaddon frowned at the interruption. "But I think we should leave behind our jetbikes, Dreadclaws, Whirlwinds, Landspeeders - pretty much all anti-grav technology, really - as well as our bikes, attack bikes, and pretty much anything else we've used so far."
Fewer helms nodded this time.
"Dude," said Lucius the Eternal, "we might need all that stuff. Some of that stuff is rad."
"Nah, I've made up my mind. Let's just go."
"Let's. Just. Go."
Abaddon waved the Talon of Horus. Its scythe-blade fingers made clickety-click sounds.
"Okay, let's just go," agreed Lucius.
Kharn wasn't so easily placated. "What about Cyclone Launchers? Because I saw those in Horus Heresy: Collected Visions, so we must be using th--"
"I feel like I'm talking to myself, here." Abaddon pointed a claw at the World Eater. It poked Kharn in the eye.
"Ow, Jesus, man. Okay, okay. We'll go."
"Yeah, that's what I thought."
Abaddon pimp-walked from the room, strutting like he owned the place.
"Hey, what about this conversion beamer?" asked Typhus. "Couldn't we use these in Rogue Trader? These are awesome. They go, like, FWOOOOOSH."
"Leave it!" Abaddon's voice called from the other room. Typhus put it down, grumbling.
Fabius Bile sort of shrugged. "So, uh, can I come with you guys? Because I was checking the passenger lists, and none of the Legions are taking their Apothecaries. I mean... don't you need us?"
Lucius patted his brother on the shoulder. "It's okay, man. We'll allude to the Apothecaries in the background text. I mean, you won't get to ever do anything, but you'll sort of be there in the fluff, y'know? A bit? Maybe?"
"Balls to this. I'm leaving the Legion. I'll make my own rules."
Kharn snorted. "Your rules will be lame for competitive play. Just watch. And people will call you Fabulous Bill."
The Traitors walked from the chamber, out to the landing pad. It was deserted.
Abaddon turned to Ahriman. "S'up?"
"Um." The Thousand Son gestured to the empty landing pad. "Where are all our gunships?"
Abaddon ignored him. "Weren't you red a minute ago?"
"We're blue now. It's... it's this whole... thing. Look, seriously, where are our Thunderhawks?"
"Oh, right. Them." Abaddon toyed with his topknot, swishing it back and forth, like a kitten with a fluffy toy. "We won't need those. We can run everywhere and stuff. Or push Rhinos out of hangers and ride them through the atmosphere. It'll be cool."
The Traitors shared a glance. This wasn't going well.
Abaddon noticed their hesitation, and sought to calm them.
"Relaaaaax. I'll invent new stuff. Like... spider robots with daemons in, and they have claws and stuff. They'll defile things. Maybe they can be called Defilotrons. It'll be sweet. And they'll have a gun on their chests, and tiny little heads.
What? Why are you looking at me like that? You just wait. We'll rock this place all to hell."