Consider a reactor that consists of two colliding beams. Should work great, right? Well, the reaction rate is minuscule, so you're wasting a whole bunch of energy in the ones that don't collide. It's a numbers game; in the sun there's a process that has a reaction rate of something like 1 in billions of years, but it's okay because there are orders of magnitude more protons in the sun.
The point of tokamaks and stellarators is that the field lines are closed, and you won't have leakage out of cusps like in mirror machines. You prop up the energy distribution with techniques like neutral beam injection and electron cyclotron resonance heating, since the ions tend to dump their energy to the electrons.
In other machines, mostly inertial confinement devices, the claim is that a pulsed mode of operation will be effective. However this remains to be shown, and some work (see Todd Rider's PhD thesis) claims to prove that such non-equilibrium schemes cannot reach breakeven.