There's a reason the classic cues endured for decades: They WORK. The melodies, the harmonies, and the sheer catchiness of most of them were hard to beat. They paired beautifully with Johnny's (later Rod's) voiceovers, too -- the cadence & flow just felt "right."
All of those are perfectly valid creative reasons. Valid, but creativity always takes a back seat to business concerns.
The newer cues simply don't work to create any sort of atmosphere or show tone. Period.
You're right, they don't, but I suspect that's part of why they're using them. Aside from the fact that they're probably cheaper, something bland and toneless would be more likely to slip past most viewers--there as "audio wallpaper," but not noticeable enough to spark and objection from anyone. It smacks of a marketing decision to make the show appeal to the broadest base possible.
STAR TREK -- Music for the original series was wonderful, and good enough for a complete CD soundtrack release a few years ago. None of the subsequent Star Trek series had music remotely as good. Boring & bland instead of melodic & memorable.
Fred Steiner and Alexander Courage composed music that was appropriate for the era. It was cheesy, it was low-cost (TOS had a notoriously low budget, and it got worse in seasons two and three), and it fit the not-too-serious tone of the show.
When TNG came along, initially Ron Jones composed music in similar vein to the classic composers...which didn't sit well with the higher-ups at Paramount, which is why Ron Jones was out after season three. His composing style is a lot better suited to cartoons (his Ducktales music is perfect) than live-action. Some of his work from TNG was good, and a lot of his season one stuff fit the tone, but from season two on when TNG came into its own his music was too much and it was distracting. He refused to tone it down despite repeated talkings-to by his bosses (including some purportedly raw arguments with Rick Berman), and so we have the tonal shift to the more subtle music in the later seasons.
TNG is widely regarded as coming into its own in its third season, and arguably it just got better as a show from then on. And I for one love the more subtle music that came from folks like Dennis McCarthy and Jay Chattaway; even David Bell's more atmospheric stuff was appropriate for the more lighthearted episodes.
LOST IN SPACE -- Same. The original series was propelled by a slew of exciting music (OK, they got lucky by hiring some guy named John Williams, but still ...) The new Netflix series? Bet you can't hum much of its music.
If you listen to any sci-fi composers discuss their work, you'll hear them all say the same thing: they only really get to shine during selected moments, like establishing shots or during scene changes. The rest of the time their job is to support the action onscreen, not distract from it by being too over-the-top. I have the OST for the new Netflix series, and even though I admittedly couldn't hum any of the cues without repeated listens, I did really enjoy the show itself. I shouldn't be so engrossed in the music that I can't pay attention to what I'm watching, so to me that means Christopher Lennertz did his job.
FAMILY FEUD -- Why isn't the current series using any of the '70s cue variations, or redux versions of them? Really boring to use just the main theme over & over again throughout each episode.
Simple--they're doing it because it's cheap. Using one cue (and not even in its entirety at that) can't be costing them a lot of money. Feud is syndicated so I could see the producers wanting to pinch pennies anytime they can. (I've said it before, with the bland daytime set the show even looks cheap to me. The lack of musical variety strikes me as just one more symptom of the problem.)
Music can make or break a show. In the case of current TPIR, it's not exactly making it.
Except...it's not exactly breaking it either. They're clearly looking to walk the line between being tonal and atmospheric like they used to be, and giving their show an appeal to the widest possible audience. They're doing the latter by removing elements that might be seen as, among other things, "too dated." I think we can see the same thing by the refurbishments they're giving the games--they're making them more period-appropriate for 2019, while still preserving the original function.