The reasons are quite simple.
The politicians are both:
1) sniveling, selfish cowards, and
2) eager for yet more votes come election time, and those new found votes will come from the illegal alien parasites they allow to remain in this country, legal citizens be damned.
Politicians are first and foremost looking out for themselves, and most have been educated or profoundly influenced in a marxist environment.
If their actions are solely governed by how many votes they can garner, then they should suck up to the 80% of voters who support Arizona and enforcement of immigration laws.