Ask Mr. Smarty Plants

Good Question!   [and sorry about taking so long to get to it]

You didn't give a lot of detail about what you are doing with these Melampodium leucanthum (Blackfoot daisy) that you killed off.  Just in that you stated that you killed a number of them - I would guess that perhaps you are caring too much for them.   Here are the Growing Contitions recommended in the Wildflower Center NPIN Database:

Water Use: Low 
Light Requirement: Sun , Part Shade 
Soil Moisture: Dry 
Soil pH: Acidic (pH<6.8) 
Soil Description: Dry, rocky, calcareous soils. Rocky, Gravelly Sandy, Limestone-based, Caliche type 
Conditions Comments: Blackfoot daisy is a sturdy, mounding plant, that will flourish in rock gardens. It is heat and drought tolerant. Good drainage is essential to its success. In late winter, older plants can be cut back halfway to keep them compact. Rich soil and abundant water will likely produce many more flowers in the short-term, but may consequently shorten the lifespan. 

So - Growing Melampodium leucanthum (Blackfoot daisy) in the general Marble Falls area looks pretty good to me, and they are officially native in Burnet County.  But the exception here is their preference for well drained soils.  Its possible that you are overwatering and/or have it in soil that is too rich.

As a similar, but slightly different thought - Mrs Smart Plants described to me that the Wildflower Center has had similar issues.  The established theory there is that nursery plants have had good planting soil and regular watering, so they are used to that.  A gentle weaning to the actual normal conditions may be needed to get them adapted.

My own plants have issues with passive neglect - - maybe yours need a bit more of that!