What is the specific reason behind this?
they can come to earth at any time but they have chosen not to do it. In a distant past it is said that the gods walked among us and other races that we will define as extraterrestrials also had contact with humanity, made exchanges or had peaceful contacts with us, but we must understand that at that time the spiritual or awareness level was extremely high, therefore advanced creatures had an interest in staying here. In ancient Indian texts it is told how the gods left before the current era began, even the seven wise sages and thousands of wise men took refuge in hidden "safe places". They obviously knew where things were going. But it is also said that they will return to the next golden age.
The gods are immortal, immortal does not necessarily mean invulnerable even if there are siddhis capable of making the physical body resistant to what would normally kill someone such as fire or other. The reason they don't come currently is because the gods are not jesus, reality doesn't work the way you stand still all your life doing nothing and miraculously someone will solve every problem and save you, humanity has to evolve and they will not interfere unless in extreme situations. Furthermore they left their seed and potential in us, it must not be forgotten that the final goal is to become god.
This current era has a meaning, if it were not for this age, which is not evil but globally presents negativity, ignorance, conflicts and various problems, humans would never understand their mistakes, going through it and experiencing it directly is a way that he has the conscience to understand these things more effectively, it is not at all necessary, but sometimes by being hit hard you can understand what you would not understand in a thousand years.
These so-called eras should not be seen solely as an external phenomenon, but also an internal, mental and spiritual one,
As to why the gods don't come to earth to save you is quite simple,
Imagine you are the richest merchant in the country, and you have a cart full of riches with you, would you ever go to a neighborhood inhabited by thieves?
Obviously not if you care about your life, this is essentially the same thing, no one forced them to went here and give us knowledge, technology and wisdom without wanting anything in return, they have already done it many times in past, so why now would they come here?
Obviously they don't deny help to anyone on the contrary, they will help you even a hundred times, those who are close to them know it, however every creature must be left to evolve on its own, if tomorrow the gods came here to tell us everything we would never learn anything.
During the golden age man knew he was god, and consequently on the outside he found himself in contact with someone at a level similar to his, if man is on the contrary in a vibratory state of ignorance he cannot expect to find yourself on the channel/frequency of the gods. This is similar to when you listen to the radio, if you always stay on the same channel you will never hear what the other channels are saying.
The fact that they would lose their divinity if they came down here makes no sense, until a few thousand years ago they were here.