Diana was a child - just 19 years old - and a sheltered child at that when she was thrown into the royal family. She was the first woman to marry into the family since the Queen Mum (the press are much, much harder on the women who marry in - they barely acknowledge the men) Since she was part of the aristo set, I think the family thought that she would just be able to get on with it and did not do anything to ease her into the life. Charles also seemed to have that feeling. There were moments when you could see some genuine affection, but they were just fundamentally a mismatch personality-wise. Again - she was 19 - she didn't have the tools to deal with this life or to work on the marriage as she had never seen that modeled in her own family. Charles was brought up to just carry one and "never complain. never explain" so he would not have to tools to help her cope and it was easier to just shut down emotionally. I could also see the extended royal family being a bit brusque and brushing off her concerns, which fed into her mental issues. I don't think she was a bad person, she was just in a rough situation and utterly without any tools to cope with it. The family seems to have learned from this, and have all mellowed and allowed emotion to creep in in the 40 years since their marriage.
They also learned the value of easing someone into the life--it is a lifelong commitment, so a few quiet years of adjustment at the beginning is but a blip over a lifetime of service. Catherine and Sophie were also given the chance to date their husbands for a long time in order to ensure that they were compatible and able to deal with the pressures of royal life.
Diana also did not have Catherine and Sophie's advantage of a strong, close family to support her behind the scenes. Sarah did not have that either -her childhood was similarly chaotic- which is why I think she and Diana were so close.
Meghan was offered that easing in. No one would have questioned if she and Harry stated that they wanted to take a few years to establish their family before jumping in as full-time working royals. But no, she had to be the "best royal who ever royaled" and hit the ground running.
Everyone in the family also recommended that they date for more than a hot minute. Let's face it, their "dating" was nothing more than a series of hookups in exotic locales. They were still in the lust phase of their relationship with no experience of the day-to-day that a long-term relationship requires. A longer dating period would have been even more important given their cultural differences. However, Meg had to make sure that she got the ring and locked him down with a baby as soon as possible. Her clock was ticking - both biologically and in Hollywood.