You only get to see the dungeons within your level range. If you exceed the max level of that dungeon they will disappear from the dungeon browser. Heroic dungeons have a stricter level requirement than normal dungeons (max and min level), so they disappear sooner.
In your case you have reached the range of WOTLK and only get to see the heroics from there. And you're too high for BC and too low for cata heroics, so they're not there. You can still access lower heroic dungeons by going there directly, but not queue for them on your own*.
Some examples:
Level 90
Level 110
* - You CAN join the other dungeons.
Go there directly. Just walk in and you can clear them (solo or with a pre made group).
Group up with someone within qualified level range (e.g. level 70 for BC heroics) and start a party sync. Let them queue up for a dungeon and you can join them. The dungeon will be scaled to match your level (wihtout affecting the other people) and you won't be able to use skills, talents, etc higher than allowed by the dungeon, so you should feel perfectly like you're in a "real" dungeon.