I agree the most likely reason is that voice calls have become much more rare. But, a novelty-wearing-off effect may also be in play. When multiple fonts first

I agree the most likely reason is that voice calls have become much more rare. But, a novelty-wearing-off effect may also be in play. When multiple fonts first became available, lots of people used lots of fonts-- the ransom note effect. Today there are more fonts than ever, but the vast majority of text is in the default proportional and monospace fonts.