Because they all are event girls aka skill lvl5/ 4 equipement slot with good stats. Above all, Dipladenia ignore whirlwind/web node with her ability and she is easy to get. The only other ones who have this are two 6* (Nerine and Christmas rose(?)) and Lilac, a 4*.
Furthermore, there is also really good girls with +10% damage and skill rate x1.2 (Justicia and Apple and soon Ornamental Kale according to wikia) and it's far better than Iris's useless canon node target boost.
Iris's +10% damage is usefull in fact (before getting better replacement) but there is still better girls to level before and anyway, there is Toad Lily with her +12% damage for 4 squad members and she is far better than Iris.

Anyway, even if a girl don't have damage boost herself, she can be really good paired with a girl who have it (like Toad Lily or Oncidium).