Because now you are a computer diver that is diving without a computer! If you planned your dive as a computer dive, ascending is what your training should have told you to do in the event of a computer failure. As soon as your computer goes belly up, you no longer have any way to track your nitrogen loading, so you no longer know how long you can safely stay at depth. Th, is is particularly true if it occurs on a deeper dive, or if the computer fails after you have already gone on prior dives that day. This is the advantage of carrying a second computer. If your primary goes, you have a backup and you can continue your dive. If not, you should be making a slow and safe ascent.
The only safe alternative to continue your dive would be if you had a contingency plan based on tables. If prior to your dive you had already made a plan based on tables, and the dive you had been executing at the time of the failure had been following that plan, you could complete your dive just as a table diver would if they entered the water without a computer in the first place. Any subsequent dives would then be continued as table based dives. This is likely going to be a much more conservative profile than what your computer would be showing you, since you wouldn't be getting any bottom time credit for time you spent at shallower depths...but much LESS conservative than ending your dive immediately.
Of course, most people I know don't plan their dives that way, and most people I see aren't wearing a watch or bottom timer in addition to their computer. That said, this is a much cheaper method of staying safe in event of a computer failure than buying an entirely redundant system...and frankly, if tech divers can execute complicated dive plans using only tables and a bottom timer, so can you. You just have to remember to plan your dives, and dive your plan.
