Stories are remade, reimagined, and rebooted because that's what people want. Familiar stories strike at something that many people find comforting.
Indeed, even when a story is ostensibly wholly-original people still seek parallels between that story and other stories, "standard motifs", etc. Often, when faced with a truly original story, some people react negatively, because there is nothing familiar for them to latch onto. A significant number of people have a seemingly programmed antipathy toward the new and different.
The most "legitimate" form of entertainment is, essentially, the presentation of a remake, night after night. While efforts are taken to keep it consistent with the "original", changes are made over time ("workshopped" even, sometimes), and actually, in the absence of changes, it is just the ultimate form of copying before resorting to recording.
Labeling something "classic" is really just a conceit, asserting that we, sitting in our own time, can make a pronouncement for all time. While there clearly can be "old stuff" that people continue to enjoy into the current-day, as well as an appreciation for something that is old simply because it provides a touchstone for a previous time, it's not necessarily universal, nor is there any reason to believe that that status conferred is due to anything other than good fortune for the old work.
There is also a danger in considering specific presentations as classics and disparaging remakes as a result: It denies the talent of-the-day the opportunity to have the open mind of their audience.
And beyond all that, the original A Christmas Carol was produced in 1908, with remakes in 1914 and 1938 (as well as others), before the 1951 remake that the OP asserted was "the" classic. I haven't seen these earlier versions, but there is no reason to believe that they were put aside for any other reason than technology had advanced (i.e., even the 1938 version was in black and white). The latest version takes advantage of the superior video and sound technology that we have available today, just as the 1951 remake took advantage of the superior video and sound technology that they had in that time.
