I.m.h.o., since you asked, here’s an amateurish attempt :

Evo feels/was betrayed, kinda like Rafael Correa with Lenin Moreno, not much seems needed to be explained here.
I unfortunately can’t judge if he’s right to say that Bolivia would be in a better shape than nowadays under his experimented leadership, that’d require a bit of research[1].

I think that Luis Arce considers that Evo is too combative, and that they almost lost a lot with Jeanine Añez, there’s enough to have been scared, at least from my point of view their recovery of power seems as miraculous as the liberation of Julian Assange.
I.d.k. if he’s wrong to believe that toning down the m.a.s. rhetoric would protect their cultural revolution, honestly i don’t think so but i don’t/can’t know for certain.
He surprisingly still has the support of D.Choquehuanca whose political engagement makes no doubt in my eyes.
Luis Arce is still criticizing Israel, or supporting China, Cuba, Venezuela, etc., so apart from being clearly less vocal than Evo the results in international relations dont seem to have changed much, it’s at least not clearly a U-turn, as it was with Lenin Moreno for example.
I can’t imagine any indigenous bolivian that would prefer Añez to Arce, but like, e.g., Lula, Evo has a long history with the population, speaking of him being a living legend is probably too much to say of any living politician, but i’d support him if i was bolivian, you don’t put aside such influential people, i.m.o.
And i spoke about Arce’s “cautiousness”, but Evo wasn’t extreme since he said that he wasn’t against anyone, not the west nor the Resistance, and how much would they need to be pro-west until we(sterners) are satisfied ?
There may be some interest over being powerful, famous, rich, or whatever, as well as some partial disdain, but less cynically i believe that Luis Arce is afraid of what the west may do if Evo returns to power, that it’s not worth it.

These are all speculations, the truth is that i haven’t got a clue either. Following their twitter accounts isn’t enough to understand their justifications, i can’t remember when was the last time that Luis Arce mentioned Evo in his tweets for example(, while Evo Morales frequently attacks Luis Arce, without once getting an answer apparently).

(I obviously think like you that Evo should be able to present himself as a candidate for presidency, and still hope that he will)

[1] : Let’s try to be a bit less lazy.
Half an hour of research didn’t bring much, the coronavirus crisis was hard, but otherwise the economic results doesn’t seem to have been particularly bad nor good.

One recurring cause of preoccupation apparently seems to be the dollar reserves, useful for importations, debt payments, or money stability, it went from 15.5B$ in 2014 to 1.7B ten years later, and it’s even worse for the liquidity(, 800M in 2022, 372M in 2023, 153M in 2024)(, a source). However, if that’s the worst i could find then it isn’t much.

Apparently another problem seems to be the diminishing exports of gaz, which according to some articles was what enabled Evo to put forward its socio-economic measures that fought poverty. They don’t seem to get as much money as before from it, but Luis Arce didn’t cancel these social programs like a true traitor would, perhaps would he have expanded them if he could, i.d.k. :

Yet, once again, nothing absolutely catastrophic, i could focus on their exportations(&nationalization) of lithium to paint a more positive light. Luis Arce obviously didn’t privatized these enterprises, but he didn’t nationalized even more, some sources seems to hint that he’s more open to public-private partnerships, but i’d be very surprised if it lead to him abandoning the nationalization of their resources, he’s after all well placed to know that it’s a part of the neo-colonial playbook.

Some sources speak about an increase of inflation or of the debt, but the former is stable, and the latter seems to have been stabilized, at least in % of the g.d.p. :

I’ve absolutely zero doubts that i’ve missed a lot of things, it’s just that sometimes you can find aberrations at the first glance, which wasn’t the case here after the first ~30-45mn of research, so n.t.r. 🤷, perhaps that Bolivia’s society and economy would indeed have been in a better shape if Evo was still president, or perhaps not, i.d.k.