It could be because I grew up with them. But imo the prequels are fanastic movies, much better than they get credit for.
Lucas is just so good at visual storytelling. There's a line from George Miller that I love -- movies are visual music. And the prequels capture that. Think of 
this scene from Revenge, really the turning point of whole saga, and it's played beautifully, with no dialogue.
And then, I think of mythology, and this idea of universal archetypes and the resampling of old material. Similar ideas, similar motifs occur--think of the universality of the flood story, for example--but they're transposed or reordered or reinterpreted to create something fresh and new. And Lucas is the best at this. It's obvious how he does this on a plot level; the destruction of the trade federation battleship mirrors the death star, the death of Qui-Gon is the death of Obi Wan; but he also does it visually. You've probably seen the compilation of 
scenes here.
This starts with the very first shot of 
TPM: the camera pans and we see a small ship approaching a large ship, moving from left to right. Compare 
ANH: we see a small ship fleeing a large ship, moving right to left. See the structure of Palpatine's chamber at the start of 
Revenge vs 
Return; the same chair, at the bottom of the stairs in 
Revenge, atop them in 
Return. There's certain shots that are identical, like Anakin watching Palpatine's use of force lightning in 
Revenge and 
Return. 
All of this is deliberate. (George Miller, incidentally, does exactly the same thing in 
Furiosa, sampling shots from both the 
Mad Max saga and world mythology; e.g., he has Achilles dragging Hector's body around the walls of Troy).
This amplifies the feeling of inevitability in the prequels, it gives it a kind of weight and heft and universality that speaks to me.
The most impressive thing about Lucas, imo, is that he manages this while never feeling derivative. 
TFA is a less skilled filmmaker trying the same thing, and you see it, and think "this is 
ANH remix". But from 
ANH itself Lucas is already just reworking existing material (
Hidden Fortress, Flash Gordon, The Good the Bad and the Ugly). In the prequels he continues this--there's material from 
Ben-Hur, for example--while simultaneously injecting his own content. But its not just derivative; its an incredibly creative work, even inspiring its own legion of derivatives. He has such a deep and broad knowledge of cinema and he combines all of these influences masterfully.