Upvote:4
Large gliders became obsolete. They were easy to detect once radars had become smaller and more common than they were during WWII. Their ability to evade radar-controlled anti-aircraft fire would have been very limited, and surface-to-air missiles would slaughter them.
Helicopters were much more versatile, could fly significant distances very low to avoid radar, and could extract troops as well as insert them. They were more useful all round, so they replaced specialist glider units.
The remaining advantage of gliders is quietness. Some special operations forces may train with hang-gliders for quiet small-scale insertions, but such operations tend not to be publicised.