They were involved in a national, taxpayer-sponsored effort. Even if some of us don't approve, we're paying for it, so we tend to cheer them on. And they were rather expensive passengers. I'm told one shuttle mission costs $1 billion.
Concerning safety, if you judge conventionally, in terms of deaths per passenger mile, the space program probably has an excellent record. Deaths per passenger minute would make it somewhat worse. I think ultimately we generally judge on the basis of lives and dollars lost per purpose accomplished. Of course it's a bit difficult to get an objective, agreed-upon, comparison of the value and purposes of lives and dollars, spaceflights, business trips and vacations. We each make our own judgements. But when an airline crashes, do we shut down our airports until we find the cause?
I think the bigger reasons for our concern are that attention of many people was on the shuttle when it exploded. But that also points to an inherent human fault. I wonder how many people have died in accidents, and gone unnoticed on a national scale, in all phases of the space program support, including driving their cars to work. We've simply added to that toll seven people, and another billion dollars, plus the cost of the shuttle itself.
There was also our suddenly disappointed expectations, especially the expectations of those close to the victims, in a venture relatively new and mysterious to all our inherited knowledge and past experience of life on earth.