The 4E design philosophy was that any monster type had some basic characteristics, but could then easily be modified to create a wide range of different stat blocks. The 4E Dungeon Master's Guide would define 7 different possible roles for monsters: artillery, brute, controller, lurker, minion, skirmisher,

and soldier. Also each monster could be of varying difficulty, from minion, to standard, elite, leader, to solo (a monster designed to be encountered alone, thus being much more powerful). The DMG had a whole chapter on designing combat encounters with groups of different monsters of the same type. Just because you saw that you were facing kobolds didn't immediately tell you what these monsters would do, and how powerful they were. You could build a whole dungeon with monsters of the same basic type, and still have a variety of different tactical encounters.

The 5E design philosophy on monsters is that a kobold is a kobold, with the only variety allowed in the Monster Manual being rolling for hitpoints. A few monsters have different types, there are two goblins on the Monster Manual and four types of drow. Years later Volo's Guide to Monsters introduced 3 more types of kobolds, but only a few races of common monsters got that treatment. Once you met a certain type of monster, let's say a gargoyle, you could "learn" its stats, and every future gargoyle you would encounter would be exactly the same. The idea was that with the help of bounded accuracy, the gargoyle would remain relevant at whatever level. Hint: It didn't work out that way.

One problem with 5th edition is that there is not enough variety to a single monster type to populate a whole dungeon. I am currently DMing Dungeon of the Mad Mage, and in the middle of the 23 dungeon levels there are 3 levels that are about two houses of drow fighting each other, one house on top, one on the bottom, and the level in the middle being a battleground. As other level of the dungeon also prominently feature large assemblies of drow, a dungeon crawl through these levels gets old pretty fast. Especially since, if played as written, every drow mage and every drow priestess always have the same set of spells (as does every "mage" NPC, etc.). The other problem is that the idea of one monster type always being the same sometimes collides with the story itself. There is a location in Dungeon of the Mad Mage where some tiny monsters have grown to 12 times their usual size; and without having a solid system of "upscaling" a monster, the altered stats presented in the module aren't working all that well.

Of course a DM can always add his own stuff: He can give different mages different spells, he can create half a dozen new varieties of gargoyle, or invent smaller or larger versions of some monster. The problem is just that the rule system doesn't give the DM any support in that. There is absolutely no advice what the CR (and thus xp) of a larger gargoyle, or a mage with a different spellcasting level would be. You have to guess the relation of size to hitpoints, strength, constitution, attack value and damage, as the rules won't tell you. 4E had official online tools to modify monsters, 5E only has a few fan-made ones (which look suspiciously like 4th edition). Virtual tabletop systems like Roll20 don't have tools for upscaling monsters (you can edit them manually), because the rules don't foresee that. So in summary, in 5th edition modifying a monster is far more work than it used to be in 4E, and that is a pity.