Although I disagree with the definition of well-behaved QFT (why are these people always insisting on taking the continuum limit ?), the reason is the following.
If one wants to take the continuum limit (that is, take the limit of an infinite cut-off non-perturbatively), while having to specify only a finite number of coupling constant at a given (finite) scale, then one needs to have a UV fixed point of the RG flow. Furthermore, to control the flow in the IR, one also needs a IR fixed point.
However, there are not that many theories that have this property. One famous example is the trajectory that links the gaussian fixed point to the Wilson-Fisher (WF) fixed point in scalar field theories for dimension less than four. Note however that it is a very special theory that does not describe any real system, even though the WF fixed point does describe second order phase transitions of a lot of systems. (That's why I don't understand why some people insists on having a "well-defined" QFT... "Ill-defined" QFTs are also useful (and I would even dare to say, more useful), since they allow to describe real systems, and compute real quantities (such as critical temperatures).)
Another remark is that the standard $\phi^4$ theory in 4D is not well-defined if interacting, and the only such theory that is a CFT is the trivial one (no interactions), which is quite boring. However, this of course does not mean that $\phi^4$ in 4D is useless (and that's why people spent decades studying it), only that insisting on a continuum limit is meaningless.
To me, the must read reference on that is arXiv:0702365, section 2.6. See also Why do we expect our theories to be independent of cutoffs?