What you're seeing in these wave releases is just what models they can produce, Mongoose is constantly playtesting new units well in advance of what gets produced, or even slated to get produced since they need to know if a unit is viable and worth a purchase before they think about producing it. Not only that, they'll want to make sure to playtest everything together as soon as possible to show various interactions between the units, designing a unit one by one and then playtesting them one by one leads to unbalanced units, same with overall faction design and playtesting.
Whereas with SST, they already have information about how well units sold and how effective they are on the table. They'd have to adapt that to the new evolution rules, but overall it's about the same amount of work.
I suspect with Evo the first few waves would just be core units being rereleased as prepaints. Only after covering the units that they want to keep in active production would they start making new units.
No, MI are't unbeatable but a victory by the bug player often involves not having anything on the table besides a few scattered bugs, with the rest underground. Sure it's neat to move around little blip markers, but not too satisfying, especially if you have a large number of warriors that you want to swarm.
My comment was mainly intended that the rules, as they stood gave MI too much of an advantage by allowing them to lay down a lot more fire during the enemy turn than they could on their own turn.
And frankly, the skinnies came out well after the initial release, well after any release anticipation had died down. On top of that the skinny army seemed very silly to many people I know. They were too cartoonish, having been based too heavily on the cartoon kids show.
Then they focused too much on making new and different types of MI, instead of making a fourth army.
Then on top of that, probably as they were testing these new armies they realized that the SST rules, as they were, were too geared towards the MI vs Bugs fight, and the more shooty firefights between MI, Skinny and the future Forth weren't as exciting.
On top of that there was the rise of more gamer friendly games, and the decline of the hobby side.
So now we have these new evolution rules, which as I've seen from my playtests of BFE, work quite well for ranged firefights.
I think one of the main reasons why BFE was released first is that SST has been out for a while, those who were going to buy the game should already have most of the miniatures they want.
Makes me think that putting out some kind of alternate history scifi with more emphasis on scifi, or perhaps some kind of fantasy game with the evolution rules may have been a better idea, especially with the problems inherent in making miniatures look like real-life counterparts.