Using some things that I already knew, combined with some research, I've attempted to come up with some sort of explanation for this.
My apologies if this is quite long, maybe I could just turn this into a thread, but I found it quite interesting, so let me know what you think.
There are probably a couple different factors. The fact that the RIAA doesn't actually track sales is one of the main ones. This is why you see albums released on major labels usually "selling" more, when in reality the major labels are just shipping more units, and that is essentially what the RIAA tracks, shipments.
This leads us to the fact that Cube released most of his material with Priority, which was quite a small label at the time, who couldn't afford to ship millions of copies just for a certification.
Before we continue, I'd like to take this time to introduce SoundScan. SoundScan is basically a very accurate way to track actual album sales by using data from stores when an album is scanned at the register. SoundScan began tracking sales in March of 1991.
Amerikkka's Most Wanted was released prior to the introduction of SoundScan. This means that there is a bit of a gap in accurate sales numbers. The good thing is that it was not certified platinum until September of 1991, 6 months after SoundScan began tracking sales.
So this means we could probably estimate that at the time Soundscan began tracking sales, Cube had moved around 800,000 units, that is if we are trusting RIAA numbers. This estimate comes from the fact that it took 3 months for AMW to go gold (500,000 units) and that it took about another year or so to sell another 500,000 units.
So from March of 1991-September 1991 we could say Cube sold 200,000 units and went platinum. The SoundScan data I have found from October of 2004 shows that since tracking began, AMW had sold about 750,000 units. If we take the 200,000 away from that we're left with 550,000 units needed to hit 2x platinum status from September 1991-Today.
Then, if we're talking Death Certificate, that was Cube's first album released after the introduction of SoundScan so this data is much more accurate.
As of October 2004, Death Certificate had sold about 1.6 million units. Based off of my previous estimates for AMW, I would be inclined to say that Death Certificate hasn't moved another 400,000 units in that time. Which means that it has not yet reached multi-platinum status either. It was certified platinum much faster than AMW but the total sales data is also more accurate as of 2004.
Although this data is from SoundScan, it should help to understand why Cube's first two albums are not RIAA certified multi-platinum. As RIAA doesn't directly track sales but rather units shipped, and the next pressing after the original releases in the United States for both AMW and Death Certificate did not come until 2003, it would not be an option financially to press/ship give or take 500,000 copies of each album over a decade after their releases. Even with more recent pressings and digital sales.
Speaking of digital sales, I don't think they have much impact in these numbers either. They certainly do factor in but there is no way they're doing much damage to those 550,000/400,000 numbers needed.
The Predator is without a doubt 2x platinum, and seems pretty accurate as far as certification date goes. Lethal Injection also should be very close, as it only needed 150,000 more copies as of 2004.
That's about it, I tried to shorten this as much as possible, and I did remove some other information. But if you have any other questions just let me know.
Also, this is just my theory/take on it, nothing definitive.
Death Certificate is his strongest album and it dropped in October '91... plus Kill At Will dropped in December 90..