The main problem is that Sela has a complicated backstory. It's not the kind of thing that could be explained to uninitiated audiences in 30 seconds with a Khan-like speech. It requires knowing Yar's backstory, then what happened in Yesterday's Enterprise, then what happened with Sela in The Mind's Eye, Redemption, and Unification. It's... a lot. A good percentage of moviegoers probably don't even know who Yar is, let alone Sela.
They don't want the movies to require someone to have an extensive knowledge of TNG to be accessible. That is, I assume, the main reason why she was not considered to be included.
Generations required knowledge of the Duras sisters' gambit. TWOK required some knowledge of TOS (Khan) as well. Both did a good job at not going overboard in reintroducing them and, in both cases (esp. for Kirk's era), TV reruns certainly helped. (only General Chang eschewed the trend for a new baddie, unrelated to anyone prior to. that hit a home run.)
Trek was never the most popular franchise and was fan-driven. Most audiences would know, or with some deft dialogue would get a quick reminder to get back up to speed. It did peak in the 1990s, but it was never a blockbuster movie franchise. Even in the best of Kirk's era, which is TVH but that's something you couldn't keep going forever so they had to return to the basics.
AFAIK,
it was John Logan that discussed why they could not include Sela, or Ambassador Spock for that matter. That is had to do with how they were using the Romulans in the film that Sela was excluded. The only other characters that were considered for NEM that did not make the cut were Seven of Nine, Leah Brahms, and maybe Robin Lefler.
Or the Romulan Commander from "The Enterprise Incident" out of the blue because
reasons fanwank.
Sela should have been in the movie in some capacity. She could have been given her a Janeway-like cameo talking to Shinzon, complete with promotion to admiral and showing her to be the mastermind and the one who wants to destroy Earth and the Federation.
That alone would have helped a LOT, but nixing Shinzon would have been better.
Could have even thrown in a line that they were originally aiming to use Lore through their intermediaries, but ended up having to setting for B-4 instead (B-4 ‘s original backstory included being retrieved and passed around Pakleds, Bolians, Cardassians and Talosians, so rework that line a bit instead of cutting it). Maybe add another line that she’s fallen out of favour with the House of Duras, and end the convo with Shinzon cutting ties with her and going it alone, and leave it at that.
Going back to the article, it amounts to defining the reason as "But because of what we have to do with our featured Romulans we could not use Denise." And what did they want to do? Have some Romulans turn goody because of a baddie. Sela, if not that oversized candy bar wrapper sitting on the table, would do just fine as that catalyst. Well, Sela more so but I digress: The idea of a failed Picard clone alone is naff, never mind the transfusion treknobabble, never mind all the narrative hoops and hurdles that were far more convoluted than reintroducing a character briefly used in the TV show and Sela was memorable enough, right down to a better case for revenge against Picard. NEM is just an overloaded mess that needed far more tightening than TMP ever needed.
Either which way, shoehorning Spock in would have been a logistical nightmare, despite being integral to any peace process in "Unification". Why would Spock necessarily have the be there, anyhow? He's not the only one looking to unify and make peace and everything. Logan's spot-on with Spock not needing to be involved. The villain would have to be stronger, and Shinzon and his overstuffed background simply couldn't have been saved by anybody - indeed, Tom Hardy does a LOT with the script to try to elevate it into something better. Ignore the plotting and watch his performance, he's great. But the hoops and hurdles about being a discarded lab experiment (clone of Picard because, more reasons) thrown into a prison where he and his Reman buddies that actually cared about him (okay...) managed to build the super-dee-duperiest ship of all time, with nobody noticing any missing materials or energy emissions somewhere? With Romulus right around the corner, really? How many Romulans were quietly helping out in this caper? Apparently quite a few, except they really weren't. Tom (a saving grace for this film, IMHO) could have been better served in the movie, and the movie itself just wanted to chuck in too much unneeded character minutiae to needlessly convolute proceedings. Especially as (from recollection of a different article) the intent was to have a direct follow-up sequel, which didn't materialize because NEM bombed.