Only because he was just a simple head pretool of Astrotrain. The only part of that toy that even looked like IDW Sentinel, sculpt-wise, was just the head. It's not like they gave IDW Sentinel a dedicated mold of his own.
To be fair...I would say that Astrotrain/Sentinel figure had a bit more Sentinel worked into the sculpt than just the head. His chest was as close to Sentinel Prime's IDW design as you could from something that also had to be Astrotrain. I have no idea if they had Leader Astrotrain in the works as far back as TR but I'm sure they knew TR Astrotrain wasn't going to be the DEFINITIVE Gen Astrotrain, so that gave them room to deviate. With a Sentinel-esque torso and alternate modes that leaned into the Cybertonian instead of Astrotrain's traditional Earth-based alternate modes that a Gen release would usually show homage towards.
Meanwhile, Nova was such a unique design that anything other than a dedicated mold based on his comic design would have been a disservice to that version of the character. Sadly, he was around in the comics during a time when Hasbro (specifically Archer Era Hasbro) cared very little about what was going on in the IDW comics since they were more preoccupied with the multi-million dollar movies that were bringing in the big bucks more than anything ever before, as well as their super ambitious plans for a multimedia Aligned Continuity that was sure to take off and become the definitive face of the brand, right?
Agreed that Nova pretty much demands a unique design to do him justice (though a repaint of the Cybertron Prime design would work in a pinch)...the whole thing just strikes me as odd.
Nova had something of a comeback in IDW as the ultimate villain behind Shockwave's machinations, manipulating events from the Dead Universe. This run roughly ran alongside Devastation. Which shockingly used Nova as a character, despite being so G1 cartoon focused. I figured that meant he was on the new post-Archer team's minds but he never materialized. Not even as a Selects or Pulse idea.
Archer was...well this is one of the reasons I tend to not have the glowing view of his tenure others do. IDW's early run was a big part of the Transformers late 2000s rival for me, and I would gather a lot of others. There were so many unique designs and ideas that still grounded themselves in that marketable G1 zone while also being new, but the Transformers brand team never touched them for a good portion of that original hotness. I get that Archer and his team had priorities, but -ation IDW was a huge part of the second stage of my TF fandom. It would have been nice to see more love from the line during those years.
And then Archer went all in on Aligned which...well we all know what happened. But aside from the overall failure...to me Nova was such an interesting concept. He showed up before IDW decided all Primes prior to Optimus were jerks so he existed in this weird space. A Matrix-barer, supposedly chosen for a higher purpose, but an all-out villain. That's interesting. That's world building. That hints at a deeper meaning to the things we know as fans. And he had a cool as heck design.
Did it get him a toy? No.
Did it get him included in Aaron Archer's grand all encompassing face of the franchise forever and ever mythology? No.
tl;dr Nova Prime looks ballin' and he's actually got an interesting story. Toy plz.