Which kind of article could be "more suitable 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.