Habitable planets are in the minority, as your scientists well know. Habitable planets that can sustain life long enough for intelligent species to evolve are in an even slimmer minority. The reasons for this reduction is the number of catastrophes that can befall evolving life, setting the clock all the way back. Catastrophes are many, and involve more than cataclysms.
In the main, evolving life falls because of the tiniest enemy, microorganisms which are likewise constantly evolving. Microorganisms have more flexibility than larger animals and greater mobility than plants. They can change hosts if their original host is not available. They have fewer factors to deal with. Where a large animal must only breath air or water but most often not both, and locate and eat non-poisonous food in sufficient quantity to sustain its bulk, and find a mate and then carry and rear the young - microorganisms have an easier time of it all around. They can survive in water or air, most often, eat most anything at hand, divide to produce young and all in a day. What we are telling you is that the dinosaurs did not die because their food source failed to grow due to the gloom from dust thrown up from a meteor impact, nor did small mammals chew up their eggs faster than they could hatch. They died because they were genetically related, and were thus similarly susceptible to the same microorganisms. The killer is no longer recognizable, having mutated long, long ago into another path altogether.
It would be sheer speculation as to what the intelligent species on Earth would have been, had this not occurred. However, most habitable planets do indeed evolve reptilian species, not mammalian, as the dominant species. This is simply first come, first serve. Reptiles evolve long before mammals, and have a head start. It does not follow, however, that the intelligent species is mammalian only where the evolving reptilian species has had a catastrophe. Most planets evolving mammals as the dominant intelligent species have not had such a history. Many factors come into play, among them how difficult life is for the various species. A difficult life, while a curse at the moment, induces intelligence.