Considering buying and playing this series, as I really enjoy old point-and-click adventure games.

I'm just wondering why exactly SQ5 & 6 were rated T, more out of curiosity than anything else. Is there really something that different from the earlier installments that warrants the higher rating?

AFAIK there's nothing especially "objectionable" in SQV to warrant the T rating, but there are probably some "iffy" things in it. For instance, the game features some male-on-synthetic-female violence; specifically, when Roger is able to stop W-D40 on the planet. For that matter, the name of the planet itself -- Kizurazgubi, a corruption of "kiss your you-know-what goodbye" -- might have raised a few eyebrows. And then there's the whole Pukoid plot thread, essentially a riff on the zombie-plague genre, which is one of those things that generally earns any game an automatic bump from the "E" ratings family.

As for SQVI, some sequences such as the business where you're inside Stellar's body are what most likely did it.

There are a few 'suggestive' comments in the games. Not that many though. The 'alcahol' in the games might also have bumped it.

IIRC, SQ5 didn't even have a rating. I think it was made before ESRB ratings were developed?

Actually I had researched this subject many times since the late 90's and as of at least the last 20+ years, whether retro actively rated by the ESRB for anthology collections released in the mid to late 90's only or if SQ4, SQ5 and SQ6 have official ESRB ratings individually asserted retroactively or contemporary. SQ4 had a KA rating, SQ 5-T and SQ6 also a KA. So with confirming this from the ESRB home site these ratings were either rated due to the anthology collections, though why not give a rating to every entry in the SQ series if that was the case? Or all I can think of is SQ4&6 had CD-rom releases and possibly later versions of the speech SQ4 game released a couple years again and was rated by the ESRB? There are so many odd occurrences with this subject and besides the ESRB having official rating to each individual last three games of the series, how on Earth would SQ5 be rated Teen, yet SQ6 was rated KA and prob converted to E now? SQ6 has so much more suggestive and lurid topics with much more detailed and explicit graphics than the rest of the series. So I can understand in an odd way why SQ5 might have got a Teen rating but for SQ6 to get KA baffles me entirely. If you curious go to the ESRB site and search ratings for space quest and the last 3 games will only show up with the ratings I stated.