Short answer: Some people just like playing them. Someone people don't.
As for a semi-reasoned breakdown.
Roleplaying a religious character
Some people very much enjoy roleplaying religious characters, it serves as a very useful hook, drive and characterization of behavior.
Of course you don't *have* to play a cleric to play a religious character... but it does put it at the core of the characters identity.
So "I want to play someone very religious = cleric" does make a lot of sense.
For those who greatly dislike Alignment, the tenets of the individual faiths of the world make a far more diverse and interesting moral compass for characters.
I actually find D&D clerics as Divine Magic a little too formulaic and insufficiently miraculous for my tastes, but that's down to how D&D magic works and has always worked. 5E's "Divine Intervention" is a nod, if a small one in the right direction for me.
Paladins are knights, not priests
I've got a friend who loves playing Paladins, but doesn't get clerics despite their apparent similarities.
And got other friends (and me) who are more the opposite.
For those more from 1E and 2E AD&D, Paladins tap into the "pure champion of chivalry" rather than "warrior of the faith" archetype.
3E drew them somewhat closer together and the distinction is still somewhat blurred.
These archetypes work more for some people than others.
They're Iconic. Being one of the "core 4 classes" for the 30ish years of the game
The iconic image of D&D is a formula that maintains a draw for many people.
Bards and Druids have even more baggage in many peoples heads
The other two primary healing classes.
Bards being associated in peoples heads as minstrels pratting around with lutes and druids being wussy hippies who whine when someone cuts down a tree.
We're seeing breaking out into BATTLE SCALDS! SWASHBUCKLERS! Into the gamer mindset and people who turn into BEARS and laugh at the idea that nature needs "protecting" when it can crush civilization like a bug. But the stereotypes still persist.
The various domains offer a good degree of flexibility with character design
7 domains in the PHB (Knowledge, Life, Light, Nature, Tempest, Trickery, War), plus Death Domain in the DMG provides a broad depths of variety to customise a character mechanically. More than all other classes have subclasses.
Almost on par with the 8 Arcane traditions of the wizard and I would argue that the differences in domains is substantially more than with the Arcane Traditions
A POWERFUL SPELLCASTER!
Unlike the Paladin and the Eldritch Knight, the Cleric is a primary spellcaster with full spell progression. In terms of being a TRUE GISH, it is *arguably* the best starting point.
And (at least some domains) get to wear heavy armour and/or use martial weapons, the only primary spellcaster to do so.
They are a prepared spellcaster, so tend to have both more spells to hand and more flexibility than known spellcasters like the Bard and Sorcerer. (A flaw in my opinin)
The spell list for Paladins is quite restrictive, and Eldritch Knights know very few spells.
Wizards with Arcane Recovery and Land Druids with Natural Recovery do get a notch more spells/day, but channel divinity can arguably compensate for this.