Pure speculation but…
Sometimes pilots get fixated on a problem and forget to lower the landing gear. Gear uup landings in small piston aircraft are not uncommon.
In this case, it seems like the aircraft may have experienced a bird strike in the right engine that may have taken out one of the hydraulic systems. However, the landing gear on a 737 can be dropped manually with 3 pull handles that mechanically release the uplocks, allowing the landing gear to drop out by gravity. I cannot imagine any scenario where this simple system would have not been effective.
There may have been smoke in the cockpit or maybe the pilots were just in a panic to get the bird on the ground and forgot to manually release the landing gear. But the flaps were also not lowered. Both of these are emergency checklist items. And they deployed the engine thrust reversers immediately upon touchdown.
The CVR will answer the questions of whether they did or did not follow the checklist and/or tried to lower the gear manually.
The Tower Controller should have also likely have called out the missing landing gear unless the pilot already said they were unable to. But if that were the case, then the Emergency vehicles would have already been rolling if not already on the runway.
All of which leads me to SPECULATE that the pilots simply forgot the gear or selected the normal gear switch down and assumed the gear was down.
Another concern of mine is the runway overrun. ICAO standards call for a certain safe area, and now a soft frangible concrete overrun that slows the aircraft like those gravel runaway truck offramps in steep areas of highways.
Such a tragedy.