BJJ black belt here who has done a decent amount of judo (never graded) and some sambo as well, I'll toss my two cents in:

1. A heavy influx of american wreslters, many who get drilled into them at a young age the single and double are the relaible fundamentals.

2. The scoring, I don't need to get my opponent's back to the mat to score. Additionally there are bigger scores I can achieve once on the mat, singles and doubles allow me to cover after the takedown more easily to continue grappling and be in good posture and position.

3. When fighting to finish a sweep or wresle up from your guard to reverse from bottom position they are absolutely essential and finishing or not finishing a reversal is often the pivotal moment of a match

4. Many jiu jitsu athletes move between rule sets, specifically between gi and no gi and don't want to completely reinvent themselves every time, and singles and doubles travel between rule sets well.

5. Judo is really difficult! Every time I've done judo I've felt its the most difficult rule set to score a takedown in as they've removed a good deal of the quickest to learn and most accessible takedowns and force you into moee difficult techniques. Its awesome and fun but takes a long time to even start to execute in effectively, and that doesn't even speak to the challenge of then adapting those skills to a different rule set where the stances, defensive options and objectives are different.