Now, I would like to play devil's advocate here, and I have a few weak points, but they should still be pointed out.
-in modern media, something that tends to be written in is characters being revived for the sole reason of a reunion. This doesn't make sense, and is very frustrating, especially when they have already completed their arc and don't start a new one. They are just there and don't have a purpose . Echo was revived and he was then given a new arc. I think that they kept tech dead because they didn't want to have him revived for no core reason. CX-2 would have been a good way to give him a purpose for revival, I will agree. But, in one of my other points you will see how that wouldn't have worked because of the laws of gravity. Tech had completed his story. So they let him stay dead instead of giving him a whole new story, which I think may have been better in the end
-while I can't excuse the argument between Hunter and crosshair not bringing up the loss of tech, I do think that a good reason they didn't mention him to much is because of the fact that for a good chunk of it, they where focused on getting omega back OR omega is captive and is more focused on getting out then mourning. There is also the fact that there was a time skip in between, that could have been the time they took to mourn, and an episode like that would have not been pushing the story forward, only dwelling on the past. While it would show the characters on a more emotional level, it would have done close to nothing for the story, and if morphofan is correct, and there was supposed to be 4 seasons but the squished 3+4 together, then they would have definitely had to cut that out for time.
-echo being revived actually makes a lot of sense, mainly the fact of how he was revived. Kept alive by machines? Lost his legs and arm in an explosion? Yes, he could be revived. But the fall tech suffered? No. One of my favorite books, which gives details on the ways people died in Yellowstone National Park (the book is called death in Yellowstone) they have a section about deaths from falling/heights. Read through that chapter and you will soon learn that the fall could have easily killed him, or if he didn't die, the time it would've taken to get to him would've. He would have had around 5 minutes after he landed that he could still be alive, and I don't think that his body was found that fast. "Well they could bring him back to life!" His body would have been so mangled and destroyed that there would be no ways to same him. Genuinely.
I do have the rare opinion of thinking that Tech staying dead was the better thing to do with all of the factors, but I do agree that they should have mentioned him more, and maybe even do a little memorial. Not just the glasses.