Spinners almost always have higher strike rates than fast bowlers. This means they have to bowl for longer to take a wicket - which almost inevitably means they will concede more runs per wicket.
A fast bowler who normally opens the bowling has more things in his favour - when it comes to taking wickets - than a spin bowler. First of all, he always gets to bowl to batters who are new to the crease, a spin bowler often comes on to bowl when the batsmen have been in for quite a while. An opening bowler gets to bowl with the new ball, often on a pitch that may offer a little help for the first few overs at least. A new ball is of little value to a spinner, even if he gets to use it. Furthermore, if a team is bowled out for a low score, the spinner often doesnt get much of a bowl. When a team gets a big score, the spinners get more of a bowl. This is why, over the last 40 years, a top quality fast bowler usually has a lower average than a top quality spinner, IMO.
Up until the 1950s, top class spinners could achieve career records similar to fast bowlers (O'Reilly, Grimmett, Verity, Laker, Tayfield). However, since the implementation of covered pitches, and possibly an improvement in defensive technique against spinners, 30 has been the benchmark for a world class spinner (Gibbs, Bedi, Chandresekhar, Qadir). Anyhow, given the shockwaves Murali and Warne have created in their brilliant careers, has the benchmark for a quality spinner moved back to pre 1960s levels?