Why hasn't the writers included more aliens or other humans that settled other galaxies and found random teleporters to ither galaxies?
Because this isn't Stargate. Battletech is a universe about human conflicts and a more "grounded" technology base.
What if they found a Dyson sphere or a gate in the universe but the outer sphere didn't want to be included because they had found new technology? 🤔 I am referring to the star league vs the outer rim.
See above.
Why has technology stagnated? I noticed IE four legged mech designs, or other new weapon types? I feel from 3050 to 3150 technology should advance.
Because this is, all things considered, a game. Advancing technology would mean to constantly push out new weapons that would do the same the old ones did, but with more damage. Then we'd update armor to take the damage and would have to put out new stats for old weapons. In the end, we would have to constantly outdate existing stuff to fill the hole with new stuff that does the exact same job the old stuff did. There is nothing in this to further the game. So technology is "stagnating" to avoid this trap and allow players to still play with the stuff they bought 40 years ago. Which is one of the big unique selling points of Battletech, compared to other wargames on the market that constantly force players to buy new and better stuff that does nothing different or better.