Why did "sabrina" move to the WB from ABC...??

I think ABC wanted to phase out the whole "TGIF" programming lineup, and "Sabrina" was probably not pulling in the ratings it used to.

Thats right and ABC wanted no one "teenie" series's in it´s programme.

Why did "sabrina" move to the WB from ABC...??

"Sabrina, the Teenage Witch" was never really canceled by ABC. The network was more than willing to renew the show for a fifth season, but Viacom Productions (which produced the show) wanted to be paid $1.5 million per episode. ABC refused. It planned on waiting until the very last minute and then renewing the show for a lower licensing fee.

The WB then grabbed the show. It only paid $675, 000 per episode, but it agreed to renew the show for two years (44 episodes.)

This is the last appearance of Brad.
After this episode, "Sabrina" moved from ABC to the WB. It jumped networks due to purely money matters: ABC was more than willing to renew the show for a 5th season, but it was not willing to pay the reported $1.5 million that the "Sabrina" production company wanted. The WB then grabbed "Sabrina." The WB paid a mere $675,000 per episode, but agreed to renew the show for two years (44 episodes.)
The move to the WB had a profound impact on the future of "Sabrina." While on ABC, the show had been mainly geared towards preteens. The WB, however, wanted the show to have a more mature audience. To accomplish this, the WB fired nearly all of the "Sabrina" writers, and a new writing staff was assembled. Moreover, the WB also announced a few casting changes for the show as Martin Mull, China Shavers, and Nate Richert (for the time being) were let go.