Well, it's the LEGO Group's call which characters show up each year, so "if they wanted to" they could include whatever characters they please. Most of the factors I mentioned earlier are just possible reasons why the cast has been trimmed from nine to seven, a decision which could have come at any two heroes' expense. I don't fully understand why Stringer in particular has been omitted from sets for the past two years.
I just want Stringer back. But I can't have everything. Is there any problems with shells, etc, on that? Could they bring him back next year if they wanted to?
It's possible that his primarily black color scheme is seen as less exciting or heroic-looking than the color schemes of his teammates, and thus he doesn't sell as well as his more brightly-colored teammates. In that case, he wouldn't be too likely to return, but it's entirely at TLG's discretion. I miss him as well.
I don't know what you mean about Rocka being "star hero". Yes, he's appeared in a lot of sets, but not considerably more than Furno, Stormer, or Bulk. In fact, if you include this year's sets, Rocka only appears in eight sets total, the same as Stormer. Furno has appeared in nine sets, and Bulk in seven.
Once again, Rocka is star hero for a while. Furno XL was cool. Stormer XL was cool. Rocka XL was OK, but Surge was supposed to fight Dragon Bolt and guess who gets an awesome jetpack? Rocka! Rocka gets one mech for this wave, and Evo gets the spotlight. I like this, because Evo has been somewhat underrated in my opinion. However, Rocka gets two mechs next wave. He has to share one with Surge, and I like to think of that as punishment for defeating Surge's Brain Attack enemy. Overall, Rocka has had the most attention. He got two Savage Planet forms, one Breakout form and got to fight Black Phantom, two Brain Attack Forms and now 2.5 (or 3) Invasion from Below forms. Why is this happening? Why can't this happen to Furno or Stormer, who have been in every wave?
And if anything, Evo is the star of this year's sets, appearing in Evo Walker, Splitter Beast, Evo XL Machine, and Queen Beast. He gets machines in three of those four sets, and they include sets at all but the $15 price point. One of those sets, Evo XL Machine, is the poster child of the current wave, appearing prominently in most "Invasion from Below" ads, and chances are the same will be true of Queen Beast for the next wave.
Rocka appears in just three sets this year, and although he gets a machine in each of those three sets, one of those machines is shared with Surge and emblazoned with Surge's color scheme, not Rocka's.
As for why Rocka appears in sets so often? Probably because sets featuring him sell well. And why wouldn't they? Gold is a very popular color.
And as for why Jet Rocka rather than Surge ended up facing off against Dragon Bolt in the Brain Attack episode? The main reason is that as the biggest set in the year, Jet Rocka had to be featured somewhere, and there were no other flying villains that year. Surge still ended up appearing in an extremely prominent role in that episode, so it's not like facing off against Dragon Bolt was some special privilege that Surge was cheated out of. He was just given a different, no less significant role in its place.