Besides all the truth or legend around flying and signing NDA’s, I think there were clear economic reasons for ending up with Microsoft’s DOS, and the pre-cursor to that: picking the 8088. [1] By 1980 there were an estimated 8,000 software packages for CP/M available, many aimed at small business. IBM was targeting that. The availability of source level converters for 8080 code to 8088 code made porting economically feasible for the (cottage) ISV’s. This must have been a strong argument in favour of picking the 8088 for the original PC. [2] In line with their respective tried and tested business models, Digital Research offered CP/M-86 with a per-copy license structure. Microsoft offered QDOS with a one-off license structure. The latter was economically more attractive to IBM. I don’t think either side expected clones to happen the way they did, although they did probably factor in the appearance of non-compatible work-alikes. Although some sources suggest that going with the 68000 and/or Unix were considered, it would have left the new machine without an instant base of affordable small business applications. Speed to market was a leading paradigm for the PC's design team.