The common cold is caused by a variety of viruses and strains that identifying all the possible DNA sequences to target would overwhelm researchers for years. This treatment is incredibly targeted, which requires the exact target DNA sequence of the pathogen to be known, siRNA snippits designed and tested against that sequence, and then for the siRNA to be put into a drug.
Ebola was probably chosen because it is a rather stable virus and has only relatively few serotypes.
Actually there are over 100 strains of cold viruses, many of which were completely sequenced and reported in a paper in Science last year.
But as to why this group of scientists chose Ebola: they have been working on Ebola for over 20 years, so they made that choice long ago. Every human virus has a scientific community of experts who study it.
Ebola was probably chosen because it is a rather stable virus and has only relatively few serotypes.