Dimitri Mendeleev designed his periodic table so that elements with similar properties were grouped together and group VIII demonstrated the most similar characteristics within each element, more so than other transitional metals.
Things such as the oxidation state of +2 and +3 being common among those specific metals-this is the best reason I could find and they are more consistent with the 18 electron rule than other transitional metals, particularly Fe, Co, and Ni.
The old system was removed because we thought there was sound reasoning to Mendeleev's decision (maybe he saw things we didn't), but there wasn't, and neither did we find enough cause to keep it that way. It has only led to people being misled or distracted because their immediate thought is: "why are they together?". When really, there is no solid justification.
From what I could find, the rationalisation for it being removed wasn't discussed at length or maybe even at all, so the above paragraph is just my assumption.
p.s. I'll add to this as/if I discover more.