It wasn't always. It had some potential but it was wasted so utterly it pains to remember it.
First 4D chess move happened https://archive.is/Elik8
And everything comparatively died out because endchan was mainly disgruntled refugees from 8chan.
/am/ stayed because there wasn't really anywhere to go and for a while endchan was /am/chan.
Original /am/min 'forgot his password' TM and a recruitment for a new board owner was announced but, as it later leaked out, by the time the announcement was publicly posted the position was already handed over to someone from IRC who didn't get /am/ at all and according to leaked IRC logs headed the counsel of the single most hated and ill adjusted posters on /am/. Things escalated, a civil war broke out and /AM/ was made. When pretty much everyone moved to /AM/ Nito/enduser, the /am/min at fault for the commotion backed off from his draconian policies and promised to be a good boy but some didn't trust him (spoilers: they were later proven right) and harmful split began. One dumb /intl/ idolizing cunt from /AM/ went against the will of two other /AM/igos (tried /am/ veterans) and kept posting unfunny, spammy shit just to irk the whiny counselor and provoke Nito to use his mod powers again but all it did was bring animosity towards /AM/ in /am/igos that wanted to move on and resume normal shitposting shenanigans.
Eventually Nito gave up his throne but by then the damage was done and /aM/ lost most of its impetus which only continued to dwindle as worse and worse people seemed to join the ranks of /AM/ and refused to understand what /aM/ is about. Eventually that never ending bad phase tired out even the most hardcore freeposters of some ended up leaving even with a farewell post.
During that 2 year long decline /sp/ arrived and endchan was /sp/chan. /librejp/ was brimming with traffic too but it was Japanese board so it was largely inconsequential. Still having our very own Nips on board was a neat status symbol.
/sp/ eventually left for their own site due to downtimes and slow load times for threads. There was a lot of technical issues back then and endchan was barely usable. Administration fucking up something with the database and deleting vast majority of the images without the ability to repost the exact same file didn't help either. I think /librejp/ moved to /sp/'s site for a while but it started dying out over there too last I've checked (few years ago) so maybe they went back to Japanese imageboard sphere, who knows.
And now endchan's traffic is two or three circlejerk chat 'boards' because ¯\_(ツ)_/¯, at least the site works most of the time nowadays I guess.

TL;DR Catastrophic decisions at worst possible timings and technical difficulties that went on far too long for most people to stomach.