As far as modern films go I really don’t see any reason why the Bluray should look any different to the DCP sent to theaters, though if I had to guess, I would say that it is because the studios are probably handling the bluray releases entirely unsupervised by the directors. I wouldn’t be surprised to find out that the studios just take the DCP, apply a blanket yellow, teal, or teal and orange LUT to it, then apply some standard Edge enhancement and DNR and call it done. That way it looks like all the other modern blurays. The director probably hates it as much as we do when and if (s)he gets a look at it, but by then it’s too late, and they are busy with their next project anyway.
For older films I think it is a little more complicated. In the 1980s they would often just transfer an IP print to tape, with the levels and colors adjusted to work within the limitations of PAL/NTSC/SECAM/etc tube TVs. In the late 90’s and early '00s DVDs suddenly started selling like hot cakes and it was worth the investment by the studios to remaster as many of the old films as they could. Sometimes this involved A GOUT style rush job, where they just took the digital laserdisc master and slapped it on a DVD, while others took more care.
With the advent of HD video and bluray, digital scans from the negatives became much more common as a way to achieve the best possible picture quality - upscaling the DVDs just wasn’t good enough. However, by going back to the negatives the original color timing is lost completely and has to be recreated. Color timing digitally, rather than chemically, makes it much cheaper and easier to experiment with new “looks”. I like to imagine that the colorist for the new bluray is sat in a hi tech room with his digital workstation and a projector, screening a theatrical print of the film on a huge screen, allowing the colorist to try to match each scene as closely as possible… But I don’t think that happened very often. Perhaps they were handed the old DVD, or nothing at all. Perhaps sometimes the film’s director would supervise - but of course that doesn’t necessarily mean that the colors wouldn’t be “improved” in the remaster, made perhaps many years after the original film was released.
There is also probably some concern that the grainy, film like image will look out of place among all of the modern, blurays, shot and processed digitally from start to finish. Hence the need to scrub them clean, and sharpen them to hell.
Finally, as somebody who has tried grading scans of film prints, and encoding them, it is very easy for me to see how the addition of a fixed budget and a fixed deadline can lead to less than optimal results. Sometimes the encoder changes the colors, or the levels from your carefully created master, or you forget to change one of the settings, and a re-encode is necessary. But if the adverts have already been broadcast that this film will be available to buy on Bluray on this date - Best Buy already has the cardboard stands to display them, and the master tapes need to be sent to the factory for authoring, then your time is up… Now of course with professionals this isn’t supposed to happen, but I’m sure it does. How else do you explain the Star Wars Blurays? When the final encode was done, somebody must have piped up and said, “oh shit, these are as magenta as fuck!”, but they had to be in stores by October, so that’s what we have… Since then they have been repackaged a number of times, ample opportunities to fix the colors, but that would cut into the profits - and we bought them anyway, right?