Part of it is different codexes are made for different editions, where some mechanics are a lot different. For example, Orks were (despite design problems) quite fine for 4th ed, as most vehicles were paperbox deathtraps. You could glance tanks to death, for example. You could consolidate into new close combats, terrain automatically blocked Line of Sight...
And in other cases, GW overvalues things. They overvalue AP 3 (despite the fact you can pratfall into 4+ cover), invulnerable saves (despite ease of torrent), etc.
As for Rokkits in Boy mobs...they are cost-inefficient and would prevent the shootaboyz from, well, shooting Marines or other infantry. It's still worth taking two anyway just so you have models that can Death or Glory...because being tank-shocked into flamerback formation isn't fun when you sit there and take it...
Oh, and Warbuggies are generally a must. They're not great (they're pretty crummy in fact), but there are two reasons to use them:
1) Fast vehicles. They're more likely to get side-shots...plus you can use them to delay enemies for a turn, or inhibit opponent maneuverability...say you have a Deffrolla.Battlewagon...very few opponents would be kind enough to drive their Land Raiders straight towards the thing that can most reliably kill them! Having sacrificial shit to prevent your opponent from taking another route is worth the investment.
2) Fast Attack Slots: There's less competition here overall.