J. ornatus is certainly not a fish that I would expect to compete very well with mbuna and many peacocks and Haps. I can't really answer why your fish died, perhaps stress, not enough food... but in the end it wasn't a mix that was likely to work out long term anyway.
J. marlieri or J. regani would be better Julidochromis to mix with your fish, though depending on which mbuna you have, might not make it either.
J. marlieri or J. regani would be better Julidochromis to mix with your fish, though depending on which mbuna you have, might not make it either.