The XLT was a very inexpensive and fairly high powered sled, that propelled Polaris to nearly 40% market share with it's popularity after it's release.
It was made inexpensively by building the engine as cheaply as possible, and powering it right at the ragged edge. Then they backed off the power by using a single pipe instead of a triple pipe, instead of building the engine itself stronger to handle the power it was capable of generating.
When you add the pipes to the engine, it puts the power generation back into the area where it stresses the cheaply built internal components, and wears them out fairly quickly. You wear out the pistons, twist the cranks, and if you bore and port it you can actually start twisting the crank cases.
If you rarely use the power it can last a long time, but if you ride everywhere at WOT the engines will usually not last long.