"Not suing right away" isn't a good argument in many cases. There are myriad of reasons why people don't sue immediately. I don't think the question is worth asking.
I'm not saying it has any legal implications to have waited so long (maybe it does - I've no idea), but if this is really about ideology then the timing seems very weird.
Microsoft contributes to the python foundation, Linux, and lots of other non-profits with valuable IP. I’m sure if any of those nonprofits stopped releasing source code and began giving it only to Microsoft, there would be a lawsuit as well.
OpenAI was still effectively sharing their research until last year.
As I recall OpenAI started becoming more closed at least around the time of GPT-3 (2020). Remember them initially saying the large model was too dangerous to release?
Mr. Altman was fired for cause last year. He then demonstrated very thoroughly and publicly that OpenAI is controlled by Microsoft, and only nominally had its own board and charter.
See general allegation "C. The 2023 Breach Of The Founding Agreement".
Musk is just salty that he could not get OpenAI stocks and his Grok is going nowhere. After all he was trying to restrict OpenAI development for 6 months or something (to try to give some time to Grok).
Microsoft's investment is not the issue. The corporate change is not the issue. They were the first steps needed to create the issue Musk is targeting. Before the Altman drama, Musk probably wasn't paying attention much. Hell, most of HN didn't care either, and we live this shit every day.
If Musk had some ideological issue with OpenAI's new corporate structure then why didn't he sue right away?