In 3.5e casting time for most spells that calls forth an ally, conjure a being, or to animate undead were 1 standard action or 1 round. However now in DnD 5e that has been drastically increased to 1 minute. This is now 10 turns for a player to cast this if he was to do so in combat. I personally feel that a lot of flavor of being a necromancer or conjurer is lost from unable to bring forth your allies during combat.
So what I want to know is why the D&D 5th edition's developers decided to change these kinds of spells' casting time. If this was for balancing I would like to know what they said about what made 3.5e different from 5e that they would have to extend casting time.
I would like a citable designer statements about these design choices.