The map does not "belong" to "the Marauders". They were the ones that created it, but the lack of any protective spells on it, restricting its use to only Mooney, Wormtail, Padfoot and Prongs, means that they never intended to keep it purely for themselves. The only indicator that they are the ones that created it is the fact that their names are on the map itself.
He took out his wand, touched the parchment lightly and said, ‘I solemnly swear that I am up to no good.’
And at once, thin ink lines began to spread like a spider’s web from the point that George’s wand had touched. They joined each other, they criss-crossed, they fanned into every corner of the parchment; then words began to blossom across the top, great, curly green words, that proclaimed: Messrs Moony, Wormtail, Padfoot and Prongs Purveyors of Aids to Magical Mischief-Makers are proud to present THE MARAUDER’S MAP.
The only keywords are "I solemnly swear I am up to no good", and "Mischief managed". This means that anyone who knows what this map is, can use it; thus making them a "Marauder", which explains the "Marauder's Map". It is a map, created by a group of 4 individuals, labelled as "The Marauders", that is a tool for any future "Marauder" that finds the map, and learns its uses.
The only other reference I can think of that would identify the original 4 as the creators of the map, is the use of the title "Messrs":
plural of Mr (= title used before a man's name) used before the names of two or more men, usually in the title of a company:
Messrs Wood and Laurence, solicitors
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In this case, the "company" would be "The Marauders", who have created the tool, called "the Marauder's Map".
All that said, J.K Rowling specifically states that the map was created by, and belonged to James, Remus, Sirius and Peter (emphasis mine):
The ability of Sirius Black, Peter Pettigrew and James Potter to become, respectively, a dog, a rat and a stag, enabled them to explore the castle grounds by night undetected. The interior of the castle, meanwhile, was mapped over time with the help of James Potter’s Invisibility Cloak.
The Marauder’s Map is lasting testimony to the advanced magical ability of the four friends who included Harry Potter’s father, godfather and favourite teacher. The map they created during their time at Hogwarts appears to be a blank piece of parchment unless activated by the phrase: ‘I solemnly swear that I am up to no good,’ a phrase that, in the case of three of the four makers, should be understood as a joke. The ‘no good’ of which they wrote never denoted Dark magic, but school rule-breaking; similar bravado is evinced by their use of their own nicknames on the map (‘Messrs Moony, Wormtail, Padfoot and Prongs’).
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Edit: As you have quoted in your question, Ron refers to the original 4 as the "Marauders". This is not really anything official, just a reference to them as the map creators. Outside of this, there is no hard connection between the label, and the group.
It is less wordy than "Mooney, Wormtail, Padfoot and Prongs", and it is a fairly common practice to label groups by a common factor. I.e. "the Slytherins".
The reason this is not more widely used is simply due to the knowledge of this particular group being known to only a limited few.
