Modern bikes have a greater number of gears on the cassette on the back wheel, which in turn requires a thinner chain. Thinner chains wear quicker. It’s one of

Modern bikes have a greater number of gears on the cassette on the back wheel, which in turn requires a thinner chain. Thinner chains wear quicker. It’s one of the reasons hub gears (or, for the true hipsters as the stereotype goes, a fixie) can be so completely bombproof - the lack of dérailleur gears means the chain can be made much heavier duty & lasts much, much longer. That said, your mountain bike probably did have quite a bit of chain wear, it just hadn’t reached the point that the chain had started to skip. How many gears did it have on the cassette?