What is Barr-Beck?
It is well-attested in the category theory literature (e.g. in Mac Lane's Categories for the Working Mathematician, Chapter VI) that the well-known theorem giving necessary and sufficient conditions for monadicity of a functor is due to Jon Beck. Indeed, most category theorists I know call this "Beck's monadicity theorem".
So why do others so often call it the "Barr-Beck theorem" nowadays?
As Tim points out in the comments above, there are many variants of the monadicity theorem which give sufficient, but not generally necessary, conditions for a functor to be monadic. Several of these variants may be found in the Exercises to Section VI.7 of Mac Lane (which section is titled "Beck's Theorem"). Exercise 7 reads as follows:
- CTT (Crude Tripleability Theorem; Barr-Beck). If $G$ is CTT, prove that the comparison functor $K$ is an equivalence of categories.
(A functor $G \colon A \to B$ is said by Mac Lane to be CTT when it is has a left adjoint, it preserves and reflects all coequalizers which exist, and the category $A$ has coequalizers of all parallel pairs of arrows $f,g$ such that the pair $Gf,Gg$ has a coequalizer in $B$. Note that these conditions are much stronger than those appearing in Beck's monadicity theorem. Indeed, the composite of two CTT functors is CTT, whereas in general the composite of two monadic functors is need not be monadic.)
Note the attribution to Barr-Beck. Now, this particular theorem was cited by Deligne in Section 4.1 of his paper Catégories tannakiennes in the Grothendieck Festschrift as "Le théorème de Barr-Beck".
My theory is that the name "Barr-Beck theorem" was popularised in certain circles by Deligne's usage here, and that over time (how long?) its usage shifted in these circles to refer to Beck's precise monadicity theorem. I fear that this incorrect usage has now been set in stone by its appearance in the works of modern influential authors such as Lurie.
It looks like Alexander's answer hits the nail on the head, but I thought I might add what Barr himself says. Barr and Wells write in Toposes, Triples, and Theories (on page 121 in the "Historical Notes on Triples" section):
The next important step was the tripleableness theorem of Beck’s which in part was a generalization of Linton’s results. Variations on that theorem followed (Duskin [1969], Paré [1971]) and acquired arcane names, but they all go back to Beck and Linton. They mostly arose either because of the failure of tripleableness to be transitive or because of certain special conditions.
The "arcane names" appears to be a reference to the "vulgar tripleability theorem" and the "crude tripleability theorem" which appear on page 108.
M. Barr and J. Beck, Acyclic models and triples, Proc. Conference Categorical Algebra (La Jolla, Calif., 1965) Springer, New York, 1966, pp. 336-343.
The "triple" is an older name for a monad.
Also, Jonathan Mock Beck is the same as Jon Beck, as you can see from these lecture notes and this Wikipedia page.
Follow-up: In Beck's thesis the monadicity theorem is discussed on page 8 with reference to an unpublished note: J. Beck, The tripleableness theorems. To appear. The editor's note adds that "To our knowledge, this has not appeared. Beck’s tripleableness theorems have been exposed in M. Barr and C. Wells, Toposes, Triples and Theories."
Beck refers in the intro of his thesis to the 1966 Barr & Beck paper cited above, saying "That paper contains a summary of the present work." This might explain the later attribution to Barr & Beck jointly.