It's called Windows 10 because old programs would confuse "Windows 9" with "Windows 98"

Cut the crap! The real reason Microsoft skipped Windows 9: old programs check if the version string starts with “Windows 9” to detect 95, 98 and ME

Who would have guessed?

A quote from CNET’s anonymous source:

Microsoft dev here, the internal rumours are that early testing revealed just how many third party products that had code of the form below.

if (version.StartsWith('Windows 9')) {
  /* 95 and 98 */
} else {
  /*...*/
}

and that this was the pragmatic solution to avoid that.

Read the full article at CNET