The charitable take is that most corporations want to comply with a state's regulations because unintentional compliance violations are painful and expensive, and it is relatively easy to be confident that you are compliant as a Delaware corp.
* it's easy and well-documented - the main thing you have to remember is to check the boxes that say this is an actual company, and not a holding company for a boat (where the real tax dodging is)
* it was reported to make acquisitions easier (as the company acquiring you would either also be a Delaware corp or it would be more straightforward even if they weren't.)
If you run into some legal question somewhere down the line, investors and their lawyers will be much more comfortable with Delaware law than some other state who may not have clear language on the books and/or have never tested that particular situation in court before.
Given that, would you rather have a case tried in a court that has only tried a handful of other cases, or would you rather be in a court that has handled a mountain of cases, with lots of information as to what the law really means, as it has played out in real-life scenarios?
Being tried under a legal regime where there is a ton of past history seems a lot easier to reason about than one where there isn't much.
> It's as if one is joining a club that has rules of business conduct clearly documented.
Well, yes. The law is the law, sure, but the "documentation" is much more than just the law, as written.
Many people in this thread cite most court systems are the same yet some people choose to incorporate in Delaware. I happen to be incorporated in Massachusetts because that is where I live.
However, we were strongly told that for early stage startups, some (CA) VCs would only bother looking at CA or DE companies.
(It also doesn't hurt that most businesses also find Delaware's business law to be reasonably fair and advantageous. Musk notwithstanding, of course.)