Why did the 5th Edition designers increase casting time for most conjure / animate creature spells from 3.5 Edition? [closed]

Question

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.

Answer 1

The most straightforward answer, with a designer citation, is a tweet from Mearls, here. (thanks to Doval for that link).

Adding more creatures to the party is very, very powerful. Toned down based on past experiences. #wotcstaff

(What follows is more interpretive than what came before)

Beyond just that additional combatants is a very big impact on encounter balance, a stated design goal of the 5th edition focus on three separate "pillars" of play (exploration, roleplaying and combat). In general, "ally" spells in the 5th edition have both a longer casting time and a much longer duration than analogues in the 3.5th edition.

This tradeoff makes the 5th edition spells generally more strategic than tactical than their companion spells. The creatures summoned can be used to assist in longer-term, non-combat situations, or can be summoned well in advance of a potential combat situation (or situations, as they can last for multiple encounters). This makes a spell have utility across multiple pillars instead of being limited to just short-term combat utility.