Even if the way you organize is bottom to top, that is a hierarchy which leads some anarchists to break away and form their own anarchist factions which give way to the same problem over and over.
I'm sorry, but to suggest that the most egalitarian way to fight fascists was the cause for Anarchists leaving is... A bit absurd. What source informed you this was common?
Any form of organization of any sort, is genuinely forming a hierarchy and stops being anarchism.
Hell even forming a plan is even slight steps away from anarchy, which they understood and constantly debated over.
Again, this is showing a severe lack of knowledge of what Anarchism is. Where did you come across those ideas?
This is why many anarchists started embracing socialist tendencies
Anarchists embraced socialism/communism from the very beginning. That has been their goal for over a hundred years. I'd advise reading about Mikhail Bakunin and the First International.
Spanish anarchists specifically really relied on the concepts of situational leadership which directly contributed to their defeat and fascist and communist groups were able to make decisions while anarchist groups were stuck in a crisis of decision making.
People will say superior outside fascist forces or communist forces but the reverse to that is inferior, slow moving anarchist forces that failed at the one thing that wins wars and battles – logistics.
Could you provide the sources for that? I have not come across any source that says the Anarchist militias were slow to respond or ineffective with the meager supplies they had, where did you learn this?
Once they do, a power vacuum forms and whichever in-group holds the most power or can organize the fastest will overtake the general anarchist movement and no longer be anarchist.
In Ukraine, they were crushed not due to a power vacuum, but because their allies the Bolsheviks betrayed them completely and utterly after the White Army was on the run. This was an unprecedented event in history, it was not generally expected at that time that the Communists would become violent toward people who shared supposedly similar goals. It was not due to a power vacuum, there was no threat that emerged from the already Anarchist controlled areas.
The same in Spain, they were not destroyed from within, but from ML's betraying them sooner than expected, and ultimately by overwhelming logistical firepower from Fascists.
If we were to follow your own logic; the reason France, Poland, and all the other nations that fell to the Nazi's in WWII must've mainly been due to them being capitalist liberal democracies (when in reality, it was mainly due to modern military strategies by Germany combined with their logistical ability).