Eflin has starter type stuff, he may be a good or bad starter, but he has the full portfolio of pitches, so does Irvin. That means he can change up the third time through a lineup. His strength is weak contact which allows him to keep his pitch count down and go deep into games. Now that doesn't mean he'll succeed, but that he's just better suited to starting than relieving.
Velasquez and Pivetta have shown they are two pitch pitchers, and that's better suited to bullpen work.
De Los Santos is in the same situation, if he doesn't develop a plus secondary pitch he's headed to the pen.
Suarez on the other hand should get a shot to start at some point because he has three legitimate pitches.
Even Nola needs three pitches, when he couldn't get his changeup going, he got bombed in the 7th the other night. Hitters time your stuff, and if you can't keep them off balance with a 3rd and hopefully 4th pitch, they'll get to you.