Which kind of article could be "more suitable for a journal than for a conference"? Which kind of article could be "more suitable for a journal than for a conference"?

I just received a very curious rejection from a conference in Computer Science, where the second referee argues that:


  I remain not convinced that it is very suitable for a conference.  (...)
  I still believe it is solid work, suitable for a journal, but probably too specialized and too technical for a conference such as (...).


Beyond my own judgment (this article builds upon results published in general journals publishing the same types of results than this conference), the statement that the article is "too specialized and too technical" seems contradicted by the comments from the two other referees (who both judged the paper to be relevant to the conference):


Referee 1 find everything "certainly understandable" and "clearly written", which does not sound "specialized" nor "technical":


  The work, while not spectacular, is very solid and worthy of presentation at STACS. The problems are certainly understandable to all, and the methods are interesting.
  The paper is clearly written and I have very few editorial comments.

Referee 3 finds the results "clearly described" to the point of potentially "incremental" (but still acceptable):


  The paper is well organized, some algorithms are clearly described, and some proofs are given.
  I am far from an expert, but the result seems interesting. Maybe they could be considered incremental, but that is to be expected for such an longly studied topic.



I have seen many papers presented at conferences without ever passing the threshold to be published in a journal, and I read on Glencora Borradaile's blog (here) how she found easier sometimes to submit directly to a journal (which we will do for these results), but I had never heard of results deemed more appropriate for a journal than for a conference.
While both types of venues generally use the same assessment criteria for submissions -- novelty, technical soundness, technical depth, clarity of the description --, the weighting of these criteria  differs a bit between conferences and journals. 

This is the case because conferences have three distinguishing features: (i) a page limit, (ii) a shorter (or at least more predictable) review cycle,  and (iii) immediate feedback through discussion with an informed audience. Each of these features contributes to an increased importance of the criterion "novelty" and a decreased importance of "technical depth".

The fact that Reviewers 1 and 3 have used the words "not spectacular" and "maybe incremental" indicates that the reviewers were not convinced of a sufficient level of novelty.