Why I should NOT do container plants

I am not consistent at keeping a garden watered and rely mostly on rainfall. Henseforth, my container plants do not last throughout a hot summer. Here's a perfect example of zinnias planted in a container, and one in the ground:

I think the reason I put some in pots was to give the garden more dimention, and I thought I could squeeze the pots into spaces that had previously been taken by spring flowers that had yet to die back. I may have thought I'd be more proactive in watering but that was sadly not the case. When the hot dry summer finally cooled down, the in-ground zinnia bloomed to awesome beauty while the one in pot died. It was such a contrast! The heat was so tremendous, we couldn't even get sunflowers to srout and grow from seed. Marigolds refused to budge until September! I'm afraid they won't have enough time to grow flowers before Day of the Dead.