ThePilot4ever, DT is always open for cooperation with 3rd party modelers, willing to contribute their creations for the official add-ons. However, their models, maps and what not should comply with 1C:Maddox and DT technical and quality requirements, which not always true for the planes and other objects, released as mods. Some authors are wiling to rework their models, but this takes quite some time, some other - have no desire, or time to do that. That is why it often takes much longer for a new plane to appear in the official release, then in a mod. And that is why some objects from mods do not get in at all. And the third reason for a mod not to get into the official release, is the fact that some mod authors do not want to see their work in the DT add-ons.
Next, DT, releasing official add-ons, has to follow certain rules, which would not allow inclusion of copyrighted material, which often get into modded IL-2 from MS Flight sim (or other games) without even notifing their original authors and owners, let alone getting their permission. Adding such stuff into the official IL-2 may bring serious leagal and financial problems to 1C and DT members. Obviously enough, DT tries to avoid at all costs getting their rears fried by lowyers.
Finally, about jet era planes. DT memebers believe, that this topic does not belong to the WWII simulation and should be kept apart, may be in a new game, or in a separate installation, even using IL-2 engine. However, DT members do not have posibility, time and desire to support more then one game at a time. So we would gladly leave it for others.