What happened in the 1980s?

I'm sure the double success of Grumpy Old Men and Glengarry Glen Ross helped in that department. The latter reminded everyone of how great he was as a dramatic actor, and he continued to be in peak form on that end throughout the decade with A Life in the Theater, Short Cuts, Hamlet, Tuesdays with Morrie, 12 Angey Men, and Inherit the Wind. Grumpy Old Men also gave Jack and Walter's team-ups a viable commercial potential, and because of that they ended up making more movies together in the '90's than they had done previously.

Also, Jack's own willingness to work may have had some bearing on his output. By 1970, Jack was arguably the most popular American actor and he may have just felt that he no longer needed to prove himself, and so only accepted parts that he couldn't turn down. By the '90's, however, his outlook may have altered to where he didn't want to be like other actors of his age and simply rest on his laurels. And it payed off, as he had what I consider to be the greatest late-career renaissance any actor ever had.

I think during the 1980s especially, he was more involved with theatre work than anything else so whenever he did accept parts in film or television, it would have been something that he would have considered worthwhile. But I agree by the time the 1990s began, he seemed back on form, movie-wise.