Extra attacks follow their own progression, making them better than base attacks.
A fighter at level 20 gets base attacks like: +20/+15/+10/+5
With an extra attack from Haste, it looks like: +20/+15/+10/+5/+20
A cleric at level 20: +15/+10/+5
A cleric at level 20 with Divine Power and Haste: +20/+15/+10/+20/+15
When adding extra attacks to non-fighter classes, those attacks are always going to be more effective than the base attack a fighter gets. While a fighter makes a fourth attack at -15, a cleric would make it at -0. Using scrolls on rogues, this could be further abused by using Divine Power and Tenser's Transformation at the same time. Considering the potential for abuse, and the fact that Divine Power is supposed to give a cleric attacks equivalent to a fighter of equal level, not better than a fighter of equal level, the bonus attacks were removed. The same logic follows for Tenser's Transformation.