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Flying wings can be made to have acceptable flying qualities without any artificial assistance. Just look at the Jim Marske glider designs.

The principal downfall of flying wings is that stability in pitch is pretty much achieved the same way as with a conventional tail, with a down force balancing out the center of gravity forward of the fulcrum of neutral point of the lifting forces, but it's all being done over the very short moment arm of the wing chord itself. In other words the "tail" has been moved forward to the trailing edge of the main wing.

There are a lot of issues that result from this, pitch sensitivity and damping issues and all that, but the biggest one from a cargo aircraft's perspective is a very narrow center of gravity range. Not a big deal on a bomber with a concentrated bomb bay load, or a glider that doesn't have to cope with loading variations, but a bigger deal on a freighter. You are forced to spread the load, and the fuselage volume, laterally, creating way more frontal area than necessary (you're in effect turning the fuselage sideways), so you end up cancelling out the drag benefit of doing away with the tail in the first place, and still end up with a "temperamental" configuration.

Cargo aircraft (outside the military) almost always started life as passenger aircraft. The ratio of active large cargo aircraft to passenger aircraft is in the single percentages. Therefore, nobody develops a pure cargo aircraft from scratch.

That does not mean that no one has tried. Especially for cargo, large flying wings have been proposed which store their cargo in containers along the wingspan - hence their name: Spanloaders. Below is an artist impression from the 1970s.

Boeing Model 759-159 distributed load freighter concept from the 1970s (picture source)

For a start, with what it costs to design and certificate a new aircraft type, if a transport craft can't be reconfigured to carry either passengers or freight it won't make it off the napkin. The conventional transports we have can be switched from cargo to passenger and back, some in just a few hours. For a non-passenger transport to compete, it would have to be much cheaper (to buy and to operate) than a multi-purpose airframe.

In addition to the other answers, a reason for the lack of flying wings in civil aviation in general is that they need to compete in an environment that has grown alongside conventional, fuselage-and-wings aircraft and is ill-suited for flying wings.

This means they need to use the same airports (turning radii, RWY widths), fit into the same parking envelopes (wingspan) and be serviced by the same ground vehicles (bay heights, wing clearances). Because redesigning an entire industry worth of ancillary equipment and infrastructure has been deemed not worth the minor efficiency gains to be had from flying wings.

Flying wings simply don't have much internal space for cargo, so they're a non-starter for cargo planes.

You mention the B-2 which will carry 18 tons of bombs. However, bombs are small and heavy: for example, a US Mark 82 bomb is essentially a 130kg (300lb) metal box filled with 90kg (200lb) of explosives. Most airline cargo isn't packed in thick, heavy metal boxes like that, so turning the B-2's bomb bay into a cargo bay wouldn't create a very useful cargo plane.

_{Which is good, because the designation C-2 is already taken. *rimshot*}

It's all about CG range and how much abuse the design can take. Take a look at the C-130 Hercules. It has a huge Hstab to cope with a wide range of CG. Really a bi-plane. So is the Chinook helicopter. Holding the table up with 4 legs (6 with a canard).

So, what do we do to get to a viable flying wing? Sweep back offers improvement in pitch stability as (with washout) you lengthen the aircraft. Control surfaces can be placed at the wing tips. Reflexed camber airfoils also help. How to cope the loss of a longer fuselage/Hstab pitch torque arm? Have the cargo bay set on a roller at CG.
Pull it forward until it tips. Secure, cargo balanced! Fuel tanks can be arranged to drain evenly. Assuming a subsonic design with near neutral static stability, it may even fly without computers.

But the all important shift in Clift with change in AOA or airspeed must be accounted for.
So a small tail, like birds have, may help build a better safety margin for the design, with or without computers. Ditto for lower aspect wings. Interestingly, a bird sweeping its wings back becomes ... a delta. Sweep them back out ... an F-111?

It is possible to reduce tail size in cargo, and passenger planes.