Why the frequency response plots (of causal filters) only show positive frequency?
This has absolutely nothing to do with causality. The frequency response of a real-valued filter (i.e., one with a real-valued impulse response) is (conjugate) symmetric, i.e., the negative frequencies are redundant. That's why it is sufficient to show the frequency response at non-negative frequencies only.

You can easily see that symmetry as follows. The frequency response of a system with impulse response $h(t)$ is given by

$$H(\omega)=\int_{-\infty}^{\infty}h(t)e^{-j\omega t}dt\tag{1}$$

If $h(t)$ is real-valued, taking the complex conjugate of $(1)$ gives

$$H^*(\omega)=\int_{-\infty}^{\infty}h^*(t)e^{j\omega t}dt=\int_{-\infty}^{\infty}h(t)e^{j\omega t}dt=H(-\omega)\tag{2}$$

So, for real-valued $h(t)$, we have

$$H(\omega)=H^*(-\omega)\tag{3}$$

With $H(\omega)=|H(\omega)|e^{j\phi(\omega)}$ we get

$$|H(\omega)|=|H(-\omega)|\tag{4}$$

i.e., the magnitude of the frequency response is an even function of frequency, and

$$\phi(\omega)=-\phi(-\omega)\tag{5}$$

i.e., the phase is an odd function of frequency.

The same is true for discrete-time systems.