The only reason people in OCG/TCG use Kaiju over Lava Golem, is because Lava Golem has no support, and no one bother to stall for 8 turns when other cards can finish the duel faster. The second reason in particular, doesn't apply in DL because DL only has 4000 LP instead of 8000, making it only 4 turns to 🔥 the opponent, which is fast enough.
Kaijus don't prevent you Summoning in the same turn, which is a big deal. Also, the primary reason people use Lava Golem over a Kaiju is because it Tributes 2 monsters instead of 1. (It's still much less common than both Kaijus and Ra Sphere Mode, however.) The fact it's not a Kaiju is usually only relevant for getting rid of Gameciel.
I'm surprised we haven't gotten Santa Claws in DL yet.
Kaiju’s have a lot going over Lava Golem. No restriction of wasting your normal summon, can also summon one to your field for free after giving one to your opponent, and really powerful support (Interrupted Kaiju Slumber namely) that really seals the deal.