That is easy to answer,they lack in their slot mechanics game play a lot compared to providers like Pragmatic,Play n Go and No Limit City for example.They usually are not that creative with their slots and that is why they are not listed in the top 100 for example in Canadian lobbies.
Play n Go releases a new game almost every 10 days or sometimes even sooner,most of the time every new slot released brings something new to the table and it is up to the player if they like this or not,Endorphina on the other hand keeps bringing us only really small changes in their slots and that is the main reason they are not where they claim to be.
I think you said it all! I played some Endorphina slots, so I can say I tried them, and for sure we can't compare them with some other slot providers... you named some of them, but the list is long. Betsoft, QuickSpin, Push, BGaming, Hacksaw, Red Tiger, NetEnt, and many other providers have awesome lists of slots!
I didn't know that Play'n Go releases so many slots... I stick to some favorite ones but looks like they have over 250 slots already (just checked a bit more about their games).