In my mind there's a very elaborate, multi-page post about the evolution of RTS design over the years.
To try to distill it down to the barest concepts, RTS is about pushing little groups of soldiers around a board that have essentially no agency of their own. Without direct instruction they remain where you left them, waiting for you to push them around again. Therefore immediately there is a control limit as to how many units you can feasibly manage. At the same time there is a strategic limit as to what those units can feasibly do, because a typical RTS game play through - the way RTS has evolved over the years - is a careful balancing act between increasing your economy and increasing your military. Therefore neither military not economic strategies can be decisive, only additive. In other words, i can't beat your army of 100 units with my 10 units, over and over again, because I'm using some uber tactical skill or strategic insight - that would mean the economic part of the game would be made irrelevant.
So you can't really make complex strategic concepts possible because it would break the economy. You have to constrain economic expansion in various ways or else it threatens to increase exponentially, letting the other player simply overwhelm with sheer numbers.
All of these skill factors trickle down into typical RTS concepts, like macro vs micro, or hard vs soft counters, ect. Ultimately though the real resource is simply time and player attention.
The result of this is that RTS is a very lonely, hyper aggressive, hyper competitive gameplay that develops a very long, complex bell curve with a long, slowly descending tail of skill level. That was its hook back in the day.
But the result of this is that RTS games are just not fun to play in multiplayer. The purest, and most successful, RTS environment is 1on1, and 1on1 is rapidly a dying breed of gameplay. Team based games are where the future is at, not just because of the obvious reasons, but because in team MOBA or team-based shooter games mediocre players who only play to even against their opponents can still have a good time and hold the line while their one or two good teammates be the tiebreakers and win the game for the team. In RTS the exact opposite is true; because there is such a tight relationship between economy and military, in team games, a single weak player will bring down the whole team, and thus the larger the team the better the chances of one team having a weak player. So everyone gets frustrated and no one likes to play.
I'm half asleep but hopefully that's the gist of things. That the real reason the MOBA style evolution of RTS games "won" the evolutionary lottery. In a sense saying RTS has 'died' is probably needlessly antagonistic. To use a more evolutionary framed term, MOBAs are "more suitable" to the needs of players than RTS games. Just like the platypus or the horseshoe crab, sure, they do their thing and you can play them and enjoy them, but gaming has moved on, and there are lots of games competing for your time. Most players today would prefer to play a MOBA to an RTS. You can still make flight sims, or sub sims, and you can still make RTSs. And you can also still see horseshoe crabs.