Nothing to add here, but it is mandatory, so...
Why don't our Gods and Goddesses prevent Kali Yuga from happening?
Because it's part of the plan and leela of Brahman, and the adharma in kali yuga is actually sanctioned and brought about by Brahman. This is all part of Brahman's leela (divine play). The yugas of satya, treta, dwapara, and kali all arrive and disappear at specific times managed and coordinated by Brahman.
Kaushitaki Upanishad - For he, Brahman, makes him, a soul, whom he wishes to lead up from these worlds, do a good deed; and the same makes him, whom he wishes to lead down from these worlds, do a bad deed.
Your question can really be reduced to, "Why does God allow bad things to happen?". And one answer is given in the Bhagavata Purana:
7.1.8 - When the sattvaguna is prominent, the rishis and devatas flourish with the help of that quality, with which they are infused and surcharged by Brahman. Similarly, when rajoguna is prominent the asuras flourish, and when tamoguna is prominent the Yakṣas and Rākṣasas flourish. Brahman is present in everyone’s heart, fostering the reactions of sattva-guṇa, rajo-guṇa and tamo-guṇa.
So good and bad actions are actually caused by the Lord, in accordance with the free will of Jivas. In Kali yuga, the Lord Brahman causes the adharma of humans according to their free will.