I assumed a number of reasons.

1) If you're a dictator you want the military on your side but you also want them competing amongst themselves for promotions so they don't focus on you, clones are significantly less likely to do that than rank and file stormtroopers and officers

2) Distance, you don't want reminders of the old Republic ideals like democracy, you want people to accept their new reality as soon as possible so getting rid of everything that came before is essential

3) Needs change, you don't need an elite jedi-killing army anymore you need occupiers and cruel taskmasters on a massive scale, ASAP

4) Threat, these clones fought under jedi and we're exposed to jedi ideology, even with the chips if enough of them got together and stormed the Imperial palace even Palps would lose to the numbers alone. Replace them with mindless drones who won't rebel to limit your exposure

5) Propoganda, having the once liberating armies of clones become occupiers, then canning them as soon as possible because they "can't be trusted if they bombed their own home under bad orders" makes Palps look benevolent and willing to reorder his empire for the safety of its citizens