Hi dude,
A multitude of reasons really:
I was able to collect parts as and when I could afford them rather than a bulk outlay on an engine conversion.
Rather than spending £3k+ on a used GTE engine, I rather spend a percentage of that on refreshing an engine so that i knew that I was as healthy as could be.
Having been out in both a TT BPU and a NA-T (within 20mins of each other), I much preferred the drama and experience of the NA-T.
I wanted a challenge and wanted to learn along the way while be able to have a handle on the conversion myself.
I could personalise the drive of the car to suit me, the power delivery, output, etc.
Among many other reasons. Cost was not one of them.