Why does Naruto have so many filler episodes?

List of filler episodes:

057-071

090-112

144-151

170-171

176-196

223-228+

List of canon scenes mixed with filler episodes:

71: 18:10 to 21:10

90: 5:54 to 9:58

92: 13:17 to 14:11

112: 16:40 to 18:50

176: 1:54 to 2:12

178: 18:04 to 21:04

179: 0:00 to 4:40

180: 0:00 to 2:12

181: 18:04 to 21:04

Note: Times may be slightly off, depending on whose release you are watching.

If you want some full detailed list of filler episodes check this web:

http://www.narutoforums.com/showthread.php?t=501381

Update:

223-242,

257-260,

271,

279-281,

284-295,

303-320

source: animefillerlist.com