I am guessing there isn't sufficient slack in their fleet to be able to hand one aircraft going out of service, and it (and its crew, together with any other crews that were supposed to be working the consequently-affected flights) being out of position for most of a busy travelling day. As such, this would have follow-on effects on other flights later that day, as they have to cancel flights due to a shortage of available aircraft.

I'm not sure what your query relates to.

If you are saying that you would have given up trying to operate your flight sooner, and so should they - well, I think the airline should not cancel until they know for sure that they cannot operate. Scheduling can be dynamic - you seem to think that all the cards had fallen into place the night before, but you are ignoring the fact that all of Friday's operations, and all of that day's backlogs, delays and difficulties, still had to be processed before they know might happen. However, with one aircraft missing for most of the day, and consequently less slack in the system, if anything is going to give, it's usually going to be one of the last flights of the day.

If you mean why couldn't they get a crew on time, and why did they keep looking until the last moment - well, again, it's similar to the above. They clearly had expected some crew to be available for the run up to Scotland, but given the constraints they were under, this must have proved impossible at short notice.