It’s a combination of several factors, but a big one is taking a small guaranteed profit in the name of reducing risk.
For example, Paramount (which is owned by Viacom, the same company as Nick) shot The Cloverfield Paradox as a theatrical film but when the final product turned out poor they decided to sell it to Netflix and collect a check upfront rather than risk the possibility of a flop. If the movie had somehow been a hit in theaters they could have possibly made much more money than they ended up making, but there was enough uncertainty that the less risky move made sense to them.
Nick sold Netflix the Invader Zim and Rocko movies because the on-air underperformance of Hey Arnold: The Jungle Movie led them to believe they would not be successful enough in the ratings to turn the kind of profit they want, and Glitch Techs got sold off because it’s both the project of a previous regime (therefore the execs have no attachment to it) and doesn’t fit into the vision the current heads have for the network (comedy shows along the lines of Spongebob and TLH, especially reboots of things in that vein that worked in the past). They’re both not confident in the show and also would serve to be embarrassed if something they’ve heavily mistreated and goes contrary to their “play it safe” agenda ended up being successful, so they took the Netflix check to cut their losses and are probably actively hoping that it gets buried and *isn’t* successful