Nostalgia. Something you watched as a kid that you used to enjoy.
I don't watch it hardly at all anymore. I stopped watching it weekly when i was about 15. I graduated to being obsessed with mma stuff then.
But, Wrestlemania is like the super bowl of wrestling. Its worth the $10 i pay for a one month subscription for nostalgia purposes. (This year it was free, because wwe gave me wwe network free for 4 months to entice me to come back).
Also, when i have a kid, if my kid gets into it, another reason i would watch it is to bond with them.