Who says they are?
The SMLE action is probably the fastest-to-operate turnbolt MBR extant. As a milsurp collector, the SMLE is far more fun to shoot than a Mauser. In military trim, it is a very well balanced rifle as well and mounts very quickly as a result.
However, most of my SMLEs do not hold a candle to the bulk of my M98s when it comes to accuracy.
There are also other factors that may sway desirability. Because the SMLE headspaces on the cartridge rim, and most surplus SMLEs have long chambers, SMLEs have a reputation for being hard on brass, which would deter some reloaders. The Mauser headspaces on the cartridge shoulder, and the Mauser action does not stretch as much as the SMLE, so Mausers have a reputation for being easy on brass. Also, the bolt face on the Mauser covers a multitude of territory when it comes to cartridge selection, and, while the SMLE could be easier to accommodate different cartridges if there was an aftermarket source of the SMLE removable bolt head, no one is providing those bolt heads.
Another factor is the two piece SMLE stock and its protruding magazine. This tends to put off people who sporterize surplus rifles because it is a lot more work to make an SMLE visually appealing as a sporter than it is a Mauser. And there is a school of thought, that the non-rotating claw extractor on the Mauser is a most desirable feature.
One thing is for sure, you don't see near as many Mausers in some parts of the world as you do SMLEs, so in some sense, popularity is a geographical thing, primarily based on whether or not the country was a former British colony or a former German colony.
For me, I am happy with either one.
My favorite Mauser of all is the Czech Vz 24, both as a milsurp and as a base for sporterizing.