I've already answered this in your previous question (see last paragraph), but I'll try again.
The problem regularizing with sum(θ)
is that you may have θ parameters that cancel each other
Example:
θ_1 = +1000000
θ_2 = -1000001
The sum(θ)
here is +1000000 -1000001 = -1 which is small
The sum(θ²)
is 1000000² + (-1000001)² which is very big.
If you use sum(θ)
you may end up without regularization (which was the goal) because of large θ values that escaped the regularization because the terms cancel each other out.
You may use sum(|θ|)
depending on your search/optimisation algorithm. But I know θ² (L2 norm) to be popular and works well with gradient descent.