Roland starts back at the beginning because of what he did to

Jake.

Jake's death in The Gunslinger is one of Roland's great moral failures. Roland literally lets Jake fall to his death so he can pursue the Man in Black. And Jake eventually dies again sacrificing himself to save Roland and the ka-tet.

Roland's stuck in a karmic cycle; sound familiar.

Dharma & Karma..."Two traded pilots twixt the dangerous shores of will and judgment..."
(whutchu know about Troilus & Cressida by Billy Shakes).

Roland chooses the Tower → sacrifices others → loses Jake → reaches the Tower → fails → reincarnates → gets another chance.

Roland is reincarnated to learn why he keeps sacrificing people like Jake.

Roland starts back at the beginning because of what he did to Jake. Duh.

The Tower is a karmic machine; it won't end until Roland reaches it without sacrificing his humanity. Remember it was humanity that beat the AI on the train.

When Roland begins again, his circumstances have changed; Horn of Eld. He didn't have it at the beginning of the previous cycle. That means Roland has changed, however slightly. If he keeps making different choices, he may eventually break the cycle.

Roland remembers almost nothing between cycles, while the reader realizes that this isn't merely a physical reincarnation. It's a moral repetition.