Jirachi became part Steel because it's body is said to be made of extraterrestrial metals (Yes it came from space, refer to Wish Maker Movie.) The Flying-Normal trend began with Gen. 1. This was to keep Flying types at bay because few good Electric, Rock, and Ice moves existed. Trapinch and it's evolutions are part Ground (in the case of Trapinch, full Ground) because they are based on an insect called the Ant-Lion (To be honest, I thought they were part Bug too) Gyarados is part Flying and not part Dragon because during Gen 1, only 1 Dragon-type move existed (Dragon Rage) which only dealt little damage. As we see in Kingdra and Palkia, their typing caused them to only have 1 weakness to the Dragon-type, hence to keep Gyarados from being too powerful, made him part Flying instead. Lucaro is part Steel because he is based on the statues of Anubis that are placed around Egyptian mummification areas (They usually hold metal weapons, hence the typing) As for Pinsir, I'm not sure.