Tagetes, French Marigold, Marigold 'Durango Red'

I live in zone 10a and just planted 6 small Durango Red Marigolds amongst my well established Mona Lavender, Geraniums and Lantana in win...Read Moredow boxes, facing west. They only get direct sun in the mid-afternoon and evening.
I had originally planted French Marigolds (not sure which kind) in these boxes and they grew fast with plenty of blooms, but then withered and died despite valiant efforts to save them. I believe it was whiteflies that killed them. I’ve been told by my local nursery that they don’t ever stock Marigolds anymore because they just can’t keep them alive here. But I LOVE the color that they added to my window boxes and I’m giving it another go. Anyone know why these “easy to grow” beauties shouldn’t thrive here in Southeast Florida?