I think that part of the issue is that the Darla->Angelus->Drusilla->Spike line of vamps have much stronger human components in their makeup. If they work at it, only a semblance of a darker human echo is left.
Darla and Angelus purged much of the humanity, as evidenced by the Judge not being able to burn Angelus.
Dru (perhaps because she was driven insane first?) did not or had not overcome her humanity, and subsequently Vamp!William was much like William with backbone.
Indeed, the making of spike is traced back to events soon after vamping. We got some of that in Fool For Love when Angelus confronts Spike for his change. In Lies My Parents Told Me, we find out more. Vamp!William was so very very human and immediately vamped his mother.
However, in a twist on fate, Vamp!Mother immediately had a strong vamp component and managed to inflict maximum pain almost immediately, sending Vamp!William into the rage and anger and bitterness and betrayal that forged Spike. Spike rejected his past in a very human way, and reinvented himself by hardening his heart and becoming what he pictured as a badass.
Eventually this stayed part of his persona, because detached from humanity, seeing them as meals that walk, he could keep the anger up.
However, putting in the chip changed things. He could no longer immediately feed the vamp part of his souled by continuous bloodlust. He started having longer and longer periods where he had to be more human. A vague memory of being nice and being considerate and being concerned started to emerge. Spike started to be more Vamp!William and yet, the burden of his personality forged over a hundred years of rejection made the journey slow.
Even when he thought the chip had stopped working, he tried to retreat into the easier world of bloodlust. But it was too late, had it actually been malfing, Spike could have reverted, he could have stayed behind the safe curtain, and Buffy would have killed him.
But lo and behold, it dod not work, and Spike sought out the one person who he had the most intense feelings for, the one person who could take the rage he had over not being able to take the easy path: Buffy.
But things were changing in that relationship, too. Buffy had denied her attraction to Spike, even as it grew with his becoming more human. But there was one problem, as long as that chip remained a block in their path, Buffy's unconscious could never be sure she wasn't taking advantage of a disarmed opponent.
When it was evident that the chip didn't work (at least on her), buffy and Spike spent all of the rage over their situations fighting each other, and when that was mostly spent, lust took over.
That was, however, but a step in Spike's journey. He had let real love in and that lets in the possibility of feelings of loss, betrayal, and jealousy. ..and hope. But Buffy's darkness threw him off. She would set boundary lines, then not enforce them, constantly saying no, then acquiescing.
Then she finally had to move on out of her own darkness, and set another boundary. One that he kept testing, but found solid. Then he drank. besotted with alcohol, his judgment and reflexes impaired, he tottered over to Buffy's for one more try at the old cross-the-line. Not being in a state to take the hint, he forced his drunken "affections" on Buffy. Only after she forcibly threw him off, did his besotted brain realize how close he came to actually raping Buffy.
The realization that he had done this to the woman of his affections appalled Reforming!Spike, and he left in a panic, to stew and to make penance. Maybe it was his look of horror that convinced Buffy not to stake him right there with the handle from the toilet plunger, maybe something inside her said, "he finally got the message." Perhaps she thought she deserved it for setting boundaries and not enforcing them.
Whatever the reason, Buffy let him live, which let Spike brood on the whole situation. In deep denial, he set his mind to anger and took off. Probably to LA, to a jetliner cargo bay to Africa, then to see a demon he had heard about in legend. Even facing the demon, he was in denial. But giving Buffy "what she deserves" is a two edge sword and he was given a soul.
But it only made things worse because he did not become human, he became Ensouled!Spike. Spike, being a direct growth from Vamp!William could not "revert" to being William, as Angel basically became Vamp!Liam because he had not purged his humanity, but had wallowed in it.
So it wasn't an innocent, like Liam/Angel, having to recall the horrors of his vampire deeds, it was worse, as he had no excuse...and it drove him crazy for a while, dealing with the enormity without a convenient excuse. He had done those things, now he was living with it. And it did not help that the First Evil took advantage at that point, and kept him off balance.
As long as the original root of what had prompted Vamp!William to turn into Spike was festering, the First had a means of control.
It took the Wrath Of Kahn moment in the basement and the subsequent fight with Wood in LMPTM for Spike to start the final portion of his atonement/redemption. It took a gaudy amulet and a beam of sunlight to finish it off, consuming him in the process.
Of course, this is all off the top of my head, so I don't know if it makes all that much in logical sense.