Usage of "&" to mean "and" disappeared. Why?

Of course, to me what you wrote here appears almost exactly like it does on my keyboard, and not at all like Trebuchet. I think font settings might have something to do with it.

Art Barber says, “However, one thing has always puzzled me. Writers back then used the character “&” to mean “and”. Howcum? And howcum this usage died out? I mean, you might use it in, say, a company name like “Smith & Jones”, but big-time usage has died out…You never see “We went to the movies & had a nice time”. What’s up with that?”

I’d just like to point out that the grammatically correct usage of the ‘&’ is for titles only. Thus, ‘Smith & Jones Office Supply’ would be correct but ‘We went to the movies & had a nice time’ is not. When used in a sentence, one should always use ‘and’. 'Smith and Jones went to the movies and had a nice time" is also correct.

::So ends Tikki’s little lesson of the day::

Open your nearest newspaper to the classified section and you’ll see hundreds, if not thousands of ampersands. They’re useful when you’re being charged $42 per line for a Help Wanted ad.

I suspect that you meant stylistically correct rather than grammatically correct, (which was, in fact, the point of the OP).

200 years ago we find a significant number of publications and (formal) handwritten documents that employed the ampersand and other abbreviations and ligatures. Now we do not find them in formal texts. The grammar has not changed, only the accepted style of presentation.