Sainz didn't distance himself, he got a loan but didn't beg to be released and seemingly wanted the RBR seat but trying to push out of hte program probably made them feel like he'd jump ship given the opportunity so wasn't worth investing more time in.
Kvyat didn't have any time wasted nor is he a driver despite their best efforts. Kvyat got moved up to RBR because they considered him worthy of doing so, he was plenty fast and drove well. They also got him into F1 for the same reasons. Demotion was a very painful pill to swallow but wasn't done out of malice, it was done out of seeing that Verstappen had the potential to match or maybe even beat Hamilton, Ricciardo didn't show that potential and while Kvyat had more room to improve he didn't show that level of talent either. RBR had very little choice but to promote Verstappen to keep hold of him. That's how sport goes sometimes, you can be great but someone better comes along and the team need them more than you.
For as much as I rated Kvyat's time at RBR highly and I think he did very well, after his return to TR he performed very poorly, at times miserably. He had reason to sure, but he also could have taken it on the chin and reacted better and driven great, had he done so he'd have kept his seat or maybe had the chance to move to a different team than TR. As said the demotion was tough, but these things happen, Kvyat reacting as badly as he did was while unfortunate and understandable, still his fault and it's those poor performance that led to him being replaced by Gasly/Hartley. Hartley wasn't brought up because he was better than Kvyat or they were desperate to give him a shot, it's because Kvyat was underperforming so badly at that stage that they had few other options.
kvyat is now driving very well again but had he driven like this after the demotion he'd never have left F1.