Electrons have both charge and spin. Spin polarization is needed for a spin current.
The thing is scattering by phonons. With such a scattering of spin is not conserved. And since such scattering by phonons is the main mechanism for resistance, the spin is not conserved.
In principle, if the state at the Fermi level would be spin-polarized, that is, if the electron energy is dependent on the direction of the spin, then one would expect the spin conservation when current flows. However, such materials are extremely rare. I know of only two examples of this Heusler alloys and materials based on chromium dichalcogenides such solid solutions of substitution CrxTi1-xSe2.
I try to explain in this way. Classically, electron is charged particle. Movement of this charged particle leads to current flow. This leads to macroscopic effect, since they are large number of electrons (say free electrons). By using any external device it can be measured. As explained by Alexander, in materials, due to lattice vibration (called as phonon), this movement is hindered. That's why we can see the effect of resistance in metals. Now, electron has another important property called spin (often called as quantum property). Spin may be parallel or anti-parallel. Because of this, electron inherently possess or behaves like tiny dipole moment (analogy to bar magnet). I now make use quantum mechanics to some extent. You question yourselves into two things. What happen because of this spin? Can we measure it?. Basically spinning electrons also induces a current, but you must be very careful here, since quantum mechanically, we treat spinning electrons as waves often refereed as spin waves. Theoretically, we can speak simply. For eg. two FM, one fixed magnetization and other easy rotatable magnetization can be taken. Now we have two states P and AP. You can rotate one with respect to other and make use of GMR effect. (imagine how to degree of freedom exist between parallel and anti parallel state!). Practically it is so difficult because of interaction of phonon with spin waves makes the spin effect inconsistent and it is difficult to realize or measure spin polarized state because of inconsistency spin wave (you imagine phonon modes interfere spin waves and so on) Thus why spintronic devices or spin polarized effect is difficult to realize at room temperature. When you go to cryogenic level, phonon modes ceases, no interaction with spin waves, you can have consistent spin states. Detecting spin polarized current is also possible. I hope now you can understand this in detailed manner.
Dear Rai,
The short answer is because of interactions. The interactions are of two types: Transverse (spin-orbit coupling) and longitudinal spin-spin interactions. Relatively, the rate of decoherence/dephasing because of phononic interaction is much slower than that of spin-spin interactions.
All the best,
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