Rolemaster is still actively developed and sold
TLDR: the original publisher went under due to financial troubles (caused by a variety of reasons, likely the nail in the coffin was debts to Tolkien Enterprises). They continued trying to push Rolemaster, but it wasn't enough. Then someone bought the rights and continued on with it, most recently releasing Rolemaster Unified, so it is still being developed.
History of Rolemaster
The history of Iron Crown enterprises is well documented on it's Wikipedia page.
From this, we can learn that Arms Law and other extensions like Spell Law that together would form Rolemaster were important to the inital formation of the company and during its first few years. After I.C.E. acquired the rights to publish the role playing game for J.R.R. Tolkien's Middle Earth in 1982, the Middle Earth Role Playing game outgrew Rolemaster in Economic importance for I.C.E., but that did not end the efforts behind Rolemaster and its sibling products like Spacemaster, which were actively supported with supplements into the late 90s.
Shannon Applecline in "A Brief History Of Game #9: ICE, Part Two: 1993-Present" gives a rather detailed account what led to the original I.C.E.'s bankruptcy in 2000, which meant the company could not continue to produce the line: a mix of royalty debts to Tolkien Enterprises, leading to loss of the Middle Earth license, debts from overprinting the Middle Earth Collectible Card Game, raising paper prices, folding of many of its distributors.
The article given gives a table that shows sales numbers of the Rolemaster Fantasy Roleplay, the last version the old I.C.E. released:
|RFRP Arms Law
|RFRP Creatures & Monsters
And assesses the economic feasibilty of these sales as:
(...) The sales numbers for Rolemaster Fantasy Roleplaying are pretty terrible for a set of original core book releases, probably 50%-75% what they should have been for a healthy company
These rights to I.C.E. assets were bought out of bankruptcy, and through various iterations led to Rolemaster being published under the Iron Crown Enterprises name again here, and the product line can be bought on DriveThruRPG.com with the core rules available as PDF, soft- or hardcover books.
To me it looks like that your premise is incorrect -- Rolemaster is still produced and further expanded, with a new version, Rolemaster Unified launched only recently. As to how popular it is, or how economically successful, I have no statistics.
Popularity of Rolemaster
Well -- actually I do have some statistics from this very site. Take them with caution, because of selection bias: the users of this site may not be representative for the larger roleplaying community. Still, we can look at the posts over the last year, most of them are tagged with a game system. From this I see the following distribution (may be off by one or two here or there, I did a fast manual survey):
||Questions (Sum) %
While a few of the systems in the "Other" bucket, also have 5 or more questions, Rolemaster is in the long tail where most are, with only 1 or 2 querstions to their name.
It is not that surprising that Rolemaster does not have a larger following -- outside of D&D/Pathfinder and to a much lesser extent, World of Darkness, this is pretty much the norm. Other systems of yore that were popular back then, like Call of Cthulhu, Traveller, Shadowrun or GURPS, are likewise down in the fraction-of-a-percent doldrums.