Because it's fun!
After a giant monster collapses and boils on top of her, Samus dramatically writhes in pain, and as she absorbs and overcomes the phazon she strikes an exciting pose as the player unlocks the final suit upgrade of the game.
vs
With almost zero response to the preceding events, Samus calmly stands up and goes "huh?" cutscene over.
Sometimes videogames can choose fun. She IS the best, the armor IS supercool!
Indulge in the moment!
The original cutscene was so iconic that they literally made merch of it and both poses were referenced in promo material for multiple later games.
Even if they decided that the tone of the scene needed to change, perhaps it was a bit overdramatic and a little janky, they replaced it with....
nothing. The newer scene is a flatline, no heartbeat whatsoever. There are all kinds of things you could do to tweak that scene without turning it into the grey mush that is the PAL/Remastered version. And considering they tweaked other animations in Remastered, it's a shame they left this plate of cold leftovers alone. One way or another, give Samus a pulse.
Even this at least has a heartbeat, some sort of response to the wild shit that just happened! I'd take it!