Some models never get made
Orks for example have been out for about 4 years, i think, in the current codex.
There are no models for Wazdakka Gutsmek, Old Zogwort (yeah, you can use a regular Wierdboy, but it's not the same..), Warbuggy with Rokkits (easy to convert, but still..) , Flash Gitz. (ok, they got a crappy model with a shotgun

)
Not really a problem if you like converting and have a bunch of spare bitz, but they aren't available as kits
Eldar have no Warlock/Farseer on Jetbike (and they are a a commonly used unit where i play) despite the codex having been out for ages.
SW only just got their
TWC, and Nid FINALLY got their Tervigon and Tyrannofex, but are still missing the harpy, Doom and so on.
It happens. it's annoying, but unfortunately common.