What made MS-DOS as succesfull on first PCs instead of UNIX sytems? [closed]

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http://unix.stackexchange.com – I often wondered, why DOS became so succesful on the first PCs as it's less flexible and scalable than UNIX systems and UNIX has been broadly used before the existence of DOS. Also there could have been the possibility to do Windows NT on an UNIX base but Microsoft decided to write everything from scratch. Can anyone tell me if there were any technical advantages of DOS over UNIX for PCs at that time or if all this was due to good marketing of Microsoft. (HowTos)