Honestly, as a GM, I prefer low CR creatures.
First of all, they rarely have any fluff associated with them. You can find whole chapters if not books about the elves (and their reviled edgelord cousins, the drows), vampires, beholders, celestials, demons, minotaurs, medusae, dwarves, half-anythings (not a pun on halflings, although they are also on the list), dragons… even fucking gods have an idiotic amount of lore associated with them! But who would make materials for kobolds or goblins? Why bother with fluff for creatures that are pretty much vermin when compared to all the other monsters!? I mean, you will not fight them past the level four, right?
Wrong, because there's this guy:
and he likes his underdogs. ;)
Secondly, I hate being bored as a GM. I also can't stand to see my players being bored. This is RPG and the battle should never feel like a chore or a slogfest. Balancing encounters using pure CR is oftentimes a bad idea since what should be an easy fight can turn out to be nearly impossible for some loadouts. On the other hand, what was supposed to be the big crescendo of an epic session can turn out into this:
(pictured: a cornered and utterly fucked opponent in 'epic decisive battle' where GM forgot about someone's ability that negates major advantage of said opponent).
When you have many relatively weak opponents you can easily gauge how it will work. Are the people getting curb-stomped? No problem, I'll just delay the reinforcements or outright forget about them! Party seems to be dominating the battle and there's no challenge? Well, no problem! I'll just swap these two reserve units into this bigass thing and charge them next round! When you use few opponents, it is (in my experience) harder to control the flow of battle. It's also less varied. Sure, most of the opponents are kobolds/goblins/whatever, but these guys throw javelins, these protect their sole sorcerer and this guy over there directs his unit of weasel riders. Anyone can make that into something interesting. I don't have the skill to make a fun engagement against a single opponent. Plain and simple.