Age. The problem is that electrolytic caps dry out with time. About 7 years is considered the norm. Yes, some seem to do fine much longer, some do not. Other things start after time. Transformer microvibrations can tend to short windings about then. Flaws in silicone start to cause potential for failure about then. Pots get old, switch get dirty. A rule of thumb is that when buying electronics, figure the price/value to you based on 7 years. If it lasts longer, it is free. Less, blame the manufacturer.
I am a fan of re-caped older "QUALITY" gear, but I can do the work myself. Just because it is older, does not mean it was great. Some was, some was horrid, and some new stuff is fantastic. You have to know or trust your ears for the exact units in question, not a reviewer's.