Because looking at a bowler's economy rate and strike rate gives a more accurate picture of their effectiveness, or where they might be going wrong. Average is lazy and can be misleading.
For example, someone like Mitchell Starc has an average-looking bowling average (34.09), but has actually taken wickets (one every 61.1 balls) with just as much regularity as Peter Siddle (one every 61.2 balls) so far in their respective Test careers. Not trying to claim that Starc is a better bowler than Siddle (he's not), but economy rate and strike rate just provide a fuller picture. When looking at career stats for bowlers, I always look at those two figures before anything else (with economy rate of 3.00 or less and strike rate of 60.0 or less in mind as "good" for First Class cricket, 5.00/45.00 as "good" for List A cricket, and 7.25/18.0 as "good" for T20 cricket) .