The reason why is the AK-74 not as popular as the AK-47 is because the world’s most famous assault rifle is often mistaken for AK-74. Actually, you’re far more likely to come across an AK-74 than an AK-47. The AK-47 is now quite rare as it’s been out of serial production for a long time. Let’s see what is standing between two assault rifles in head-2-head comparison, AK-74 vs. AK-47.
AK-74 is a modernized version of the AK-47
It kind of speaks for itself. And this is without mentioning thousands, millions of AKs given by the USSR to anyone who expressed anti-American views, lost, found, captured in fights, and thus spread all across the globe, among all guerrilla fighters, third-world militaries, gangs, etc. Not to mention jury-rigged AKs, made by terrorists virtually in their car workshops, Chinese copies, copies of other gun manufacturers.
Design and production of AK-74
AK-74 (Avtomat Kalashnikov 74) came later, much much later. And first and foremost, it was used by the Soviet army. In the late 70s/early 80s, the Soviet Army completely phased out good old AK(M). Right now, you won’t find a single AK/AKM issued to a Russian soldier. Everyone uses either AK-74 or AK-74M.
You can guess where all the AK/AKM from the Soviet Army go: into the arsenals to be used in the time of need (total war and lack of other weapons) and as “gifts” to other countries that supported USSR and did need help.
So AK-74 assault rifle came to the “third world weapons market” relatively late, in much smaller numbers, and millions of AKs already took the market sector for cheap as hell, but a reliable weapon. Why bother and change to a more complicated, more advanced AK-74 when good old 7.62 works just as well?
Somali pirates and worldwide terrorists
Unlike American gun fans, Somalian pirates and Taliban terrorists can’t just go and buy a new gun because they like it/it is better. They use what they have in their possession, AK and 7,62×39, not AK-74 and 5,45×39. As an addition, you have good ammo compatibility: 7,62×39 is also used by RPD/RPK machine guns, SKS carabine, a whole lot of older Soviet weapons.
I can’t say that these guys (even some third-world militaries) have sufficient enough logistics to support three different calibers at once (5,45 for “newer” stuff, 7,62 for “older” stuff, and there are also 7,62x54R for machine guns and rifles). From this point of view, it is always better to choose the most widely used gun and caliber, so you won’t be that idiot in a war zone who ran out of ammo for his unique gun, which no one else has.
AK-74 vs. AK-47: Huge difference in power and caliber
The AK-47 and the derivatives fire the 7.62x39mm round. The kinetic energy of this round is around 2100 Joules at initial speeds of 730m/s upwards. With a more powerful charge, a reworking of the barrel, and a special bullet, energies of up to 3300 Joules are possible with longer barrels.
Just to put this into perspective, this is the level of energy a 7.62x51mm NATO bullet makes. All one needs a good gunsmith, reloading skills, knowledge about ballistics, and a variable gas chamber orifice system like the one on the Yugoslavian M-70 and M-72 (AK-47 based assault rifles). Better progressive springs also help reduce recoil.
The AK-74 fires the 5.45x39mm round. The kinetic energy of this round is 1400 Joules tops, with initial speeds of up to 890m/s. It has the same issue the 5.56x45mm NATO bullets have, the high velocity with low mass. This results in bullets flying off trajectory in high grass, woods, bushes, glass, and any other obstacles quite common in most battlefields.
Spraying bursts onto the asphalt in urban areas sends bullets off target, while a 7.62x39mm burst sends debris flying right there where you pointed fire. The penetration also depends on distance, impact surface, and movement, so there are many more than simple numbers.
In short, the 7.62x39mm has shown to be an excellent caliber/round for both field use and urban areas. It is much more powerful than the 5.45x39mm (50% higher power), and there is more practical use in real combat. Hence experienced fighters prefer the AK-47, providing more bang for the buck. Literally.