Not being able to add employees fast enough isn't necessarily a sign of a good system.
The story goes that Milton Friedman was once taken to see a massive government project somewhere in Asia. Thousands of workers using shovels were building a canal. Friedman was puzzled. Why weren't there any excavators or any mechanized earth-moving equipment? A government official explained that using shovels created more jobs. Friedman's response: "Then why not use spoons instead of shovels?"
The point being that adding laborers doesn't translate into a better product. In fact, not being able to add employees fast enough and turning away clients seems like a clear sign of scaling problems, training problems, and user interface problems. And just because they're first in the KLAS rankings doesn't mean that they're any good at improving patient health.
Now that's not to say that they fill their role as hospital practice management systems. But the fact of the matter is that hospitals are big money, and attract their own types of politics and special interests. And these KLAS rankings are about just that: politics, special interests, funding, sales, and so forth. Hospitals need practice management systems, and they need a way to evaluate different systems, and Cerner and Epic have put the most work into being on the top of the pile right now. But their systems are horribly designed.
And don't get me started on Cache. It's only redeeming quality is that it's not a relational database. But seriously? MUMPS? You're saying that we should continue deploying health care systems based off MUMPS? I'm sorry, but we've learned a few things in the past 40 years since M/MUMPS was originally created. Now then, I'll concede that object and document oriented databases are the way to go with regards to databases in healthcare. And in that regard, systems based off of Cache at least have that going right for them, and is why they've got the market share. But mark my words: a vendor is going to come along with a modern HTML5 product based off of a next-generation database, and it's going to be transformative. I don't know what that database will be... Cassandra, Hadoop, BigTable, Redis, or what. But when it finally happens, it's going to be the Facebook and Google of healthcare.