(Note, I'm in the anti-spam industry, both via F/OSS (SpamAssassin) and via a proprietary solution.)
The anti-spam industry is "mature market," which means any "new" customers are stolen from a different anti-spam vendor (including home-brew SpamAssassin setups). This makes it really hard to compete, so only the biggest players are still in the mix. McAfee decided they had better opportunities to make more money, so they cut email security loose.
So we have a market that's nearly impossible to breach for a newcomer. (Note that at the enterprise level, machine learning doesn't work well because there's nobody fleshing out the ground truth, so anti-spam is separate from the explosive growth of ML techniques like CNN, though this is of course an oversimplification.)
At the same time, we have an assumption that spam is a "solved" problem, with end users being satisfied that enough of their spam is caught that they have a workable solution. Until they're phished. Then, all of a sudden, a 99.0% catch rate isn't good enough, yet at the same time, a 0.1% false positive rate (referring to legit email wrongly blocked as spam) is an even bigger problem. Learn your ROC curves: you can't have it both ways. This means that those of us left in the anti-spam world have to very carefully balance our inputs and ensure we have maximal data to train on (yet another barrier to entry for a newcomer).
Ever hear about snowshoe spam? You can think of it as anti-anti-spam. It's specifically designed to bypass spam fighting technologies. Spam fighting is a cat-and-mouse endeavor and it takes a lot of work. Frankly, it's significantly harder than anti-virus. Anti-spam rules are very different from Anti-virus definitions. Anti-spam is probabilistic, basically "reading" the content and judging it almost the way an (expert) human would, while anti-virus is procedural, looking for very specific patterns that are exclusive to a particular virus family. (Again, an oversimplification; spam fingerprints and malware heuristics both exist but are both minorities in their respective worlds for a reason.)
So, to summarize, Intel abandoned anti-spam because it's too hard, the competition is too good, and other focuses for Intel's teams would make more money. (Also remember that Intel bought McAfee for virtualization technologies first, and binary analysis capabilities second. The security part appears to be a lower priority.)