Why isn't there more variety of fish in mountain lakes?

Mountain lakes are harsh (cold), sterile environments and only a few fish species will survive - and even less, thrive.  Temperatures alone eliminate survival of all warm and coolwater fish such as bass, bluegill, catfish, and crappie. In the high-basins of Idaho, water temperatures remain cold and the lakes ice-covered for more than half the year.  When water temperatures are below 40 F, don't actively feed and grow.  Then you add a lack of productivity at high elevations (insects, plankton, nutrients), what fish are there just don't have forage available to gain much size. So, what species currently in Idaho can we stock in mountain lakes?  Rainbow trout and westslope cutthroat trout are the predominant species we stock to provide fisheries under conditions previously described.  Growth rates are slow and steady and we anticipate they will live from 7 to 9 years.  We do stock a few golden trout and Arctic grayling.  These species come from out-of-state hatcheries and are becoming more difficult to obtain on a regular basis. Many mountain lakes have brook trout.  Brook trout were stocked in Idaho mountain lakes until the 1950's and early 1960's.  They seemed to thrive under conditions found in mountain lakes and are habitat generalists when it comes to spawning.  Unfortunately, they tend to over-populate and stunt (drive their size down) and eventually become undesirable to mountain lakes anglers due to their size and appearance of a very little body with a big head .  They also out-compete cutthroat and rainbow trout so they create a one-species fishery.  They are, however, easy to catch because of their aggressive nature.  They also are known to decimate native amphibian populations.