Real Madrid have had more great players and far more success in their history
I would like to say that this perception has shifted massively in the last ten years. All the way until 2016, Ronaldo's move to Real Madrid could have been seen as a mistake from a sporting point of view, given the relative lack of trophies compared to what he could have won at United (1 league and 1 CL in 6 years. During that time, United won two leagues, were very close to winning two more and had a few great CL runs and would have likely won it at least once had he stayed).
In his first four years at Madrid, United actually outperformed Madrid in sporting terms, having won two leagues, compared to Real's one. Not until Ramos's famous equalizer in 2014 did Ronaldo really start having sporting success with them (one La Liga to show for his efforts 2009-2013 - I am not taking domestic cups into account). This was five years into his spell with them.
If we are comparing the clubs as such, Real are now almost undisputedly considered the biggest club in the world, but this is after they won the CL 5 times in 8 years. If we roll the clock back to the time when Ronaldo joined them (2009), Real had 9 European Cups, which is still more than anyone else, but six of those had been won during what I would call pre-modern football (earlier than 1970). So yeah, while they had won most continental titles out of all the other clubs, they had only three "modern" titles, their recent CL record had not been good and they were not dominant domestically. A lot of people called United the biggest club in the world at the time(finances + fan base + recent success + historical success) and this was not considered an "out-there" opinion. One would get laughed at for suggesting the same nowadays and rightly so.
What I am trying to say is that Ronaldo has helped this perception of Real Madrid shift massively, having been a key player in 4 of their recent 5 CL wins, which is also a half of their "modern day" CL wins. So, I would argue that Real Madrid fans younger than 70 have not seen a player more influential to the club's success than Ronaldo put on their shirt and that he played a key role in them being perceived the way they are perceived now.
Therefore, I would argue that, in relative terms, he is far more important to the history of Real Madrid than of Manchester United, so I think that the answer to the question asked in the OP lies in the mentality off the two clubs and fanbases. Coincidentally, I think that this mentality is one of the reasons why United have fallen so much behind over the last 15 years (the downfall started in 2009, it just got accelerated in the meantime).
