The 3rd Generation G19 has a well-deserved reputation for reliability, durability, accuracy, shootability, and ease of maintenance that the newer weapon simply cannot match. For people who use/carry weapons for serious purposes, a solid long-term record of performance in these critical areas is worth far more than a few new features which any single person may or may not be able to use to their advantage.
In time, the new 4th generation guns may develop their own enviable record, but they have started off with a few snags in the reliability department. Given this, it will probably take more time for them to develop that fine record the older models currently enjoy.
"Placement is power" -- seen in an article by Stephen A. Camp
(RIP, Mr. Camp; you will be remembered, and missed)