Class 60s, i could never work out if they were nadgered by the collapse of the coal traffic or far too many being ordered in the first place or their reliance on late 80s technology made them prematurely obsolete and complex or a combo of all/some of these factors.
They are 'higher tech', more complex, more heavy-haul capable machines than the class 66 - the work that really *needed* them declined, so it wasn't worth spending the money maintaining them all when the 66's could cover the work that was available (and the 66's were all originally leased, so had on-going lease payments to be paid, whereas the 60's were owned by EWS/DBS/DBC so there was probably little cost in not using them). The nearest equivalent to a 60 in haulage capability is probably a class 59, which are still in active use because they are simple, rugged, reliable machines - the oldest ones have spent nearly 35 years hauling some of the heaviest trains ever to run in UK.
Note that EWS ordered the 66s primarily as replacements for the freight 47s/56s/58s etc. *not* to replace the 60s.