Either because english is a hard language to replicate by computers or due to poor recordings (like in Sonika's case). but they are not so many; and others Vocaloids, like japanase Vocaloids, also have their problems, but we don't notice them because most of us don't speak japanase. For example, the "pluck" glitch (a glitch that produces a pluck sound) is found in tonio and Sonika; but also in Gackpoid and Hiyama Kiyoteru.
Also because a english voclaoid has a voicebank x5 the size of a Japanese one, even if the frequency of the glitches is the same (eg. say 1 for every 5 sounds), english ones end up with the most. You can still work round them despite this and quite often they don't come up as often as you expect.