I am hereby waking this topic from the dead because it turned up in a forum search, and I noticed that none actually gave a straight answer to the question: why is Lazarus not written in Delphi?
To answer that question, we should turn to Free Pascal which is what Lazarus is using.
So, what motivated the Free Pascal project?
Free Pascal emerged when Borland made it clear that Borland Pascal development for DOS would stop with version 7, to be replaced by a Windows-only product, which later became Delphi.
Student Florian Paul Klämpfl began developing his own compiler, written in the Turbo Pascal dialect, and produced 32-bit code for the GO32v1 DOS extender, which was used and developed by the DJ's GNU Programming Platform (DJGPP) project at that time.
Originally, the compiler was a 16-bit DOS executable compiled by Turbo Pascal. After two years, the compiler was able to compile itself and became a 32-bit executable.
Taken from
https://en.wikipedia.org/wiki/Free_Pascal#Early_yearsFree Pascal used Turbo Pascal initially, but became self-hosted two years after.
Lazarus is using Free Pascal, and has helped drive the project forward.
There is no reason to use Delphi, because that is IMO inferior in features and lacking in platform support.
And it is not Open Source, which is fairly important.
If you have the choice, always choose open source. You are then in control.