The current design of design Divine Smite (and the Smite spells) isn't particularly interactive and oddly restrictive in the current game which wants martials bouncing around the battlefield and having a litany of on hit non-action combat control options.

Similarly, the 2024 Paladin base-class doesn't interact with its own spellcasting feature, at all, and makes no effort to blend the martial ability of the Paladin with its spellcasting feature. Instead, the design of the 2024 PHB Paladin leans heavily on the wording for its new-"ish" spells:

"a bonus action that takes place immediately after you hit a creature with a Melee weapon or Unarmed Strike."

Which allows these spells to interact more easily with later (more support oriented) concentration spells (which will still have to compete with the new Glimmering Smite), but at the same time, they are oddly more restrictive in when they can be applied. A 2014 PHB Paladin could use the Thunderous Smite spell on a reaction attack; a 2024 PHB Paladin can't use a Thunderous Smite on reaction attack.

The goal of a feature like this is to allow a Paladin player to be able to entertain what Bonus-Action style of play without precluding them from using their class features, and allow for some of those quirky fantastical reaction-based smite interactions the 2014 Paladin could have, without being huddled by either a concentration recquirement or an unnecessary Bonus action requirement.

They're still casting spells, so that's limited to a single use per turn, but now a Paladin player can indulge in being a Paladin on their reaction, or with a Polar Arm without being restricted from using their smites.