Because a lot of phones are locked down by carriers that are slow to issue over-the-air updates. Even non-cellular tablets need manufacturers to issue updates before most users will apply them.
Another big reason is that you don't need to be on the latest core Android version to get a lot of the newer functionality. To get the latest iOS Mail app, you need to update your whole iOS. To get the latest Android Mail app, anyone with a reasonably modern Android version can download the latest app.
Because a lot of phones are locked down by carriers that are slow to issue over-the-air updates. Even non-cellular tablets need manufacturers to issue updates before most users will apply them.
Another big reason is that you don't need to be on the latest core Android version to get a lot of the newer functionality. To get the latest iOS Mail app, you need to update your whole iOS. To get the latest Android Mail app, anyone with a reasonably modern Android version can download the latest app.