[VRWC_minion]

Why isn't there at least two tethered space suits available on board each shuttle for external repairs ?

Maybe for the same reason a commercial flight has no parachuttes for the passengers. Cost/benefit.

[freepersup]

In fairness to all, the proper question to ask isn't a 'why wasn't something done', but a 'what can be done' question.

I believe once the orbiter program resumes there will be at least one space suit on every shuttle for inspections and/or repair.

I believe a program will be developed to make onboard repairs to damaged tiles, should such a condition be determined as being the root cause in this loss.

In the near future, I believe an escape system will be devised. Whether it is parked at the space station or launched from earth or is incorporated into a new design for a space plane, an escape contingency will be developed.