Lots of reasons.

A.He was annoying.

B.He took Fred and Velma's roles.

C.Fred,Velma and (at the beginning) Daphne weren't in major roles.

D.He really jumped the shark.Barbera once said "If this doesn't work,Scooby's dead".Boy,was he wrong.

E.Most of the monsters and ghosts were REAL.

F.He was based in part on Henery the Chicken Hawk,which was annoying as well.

I Like Scrappy

I like him in the New Scooby Doo mysteries. But I don't like these shorts where the monsters are real (and 13 ghosts too because of the same reason). I liked zombie island with real monsters though.

I loved the kid except in season 1 his voice didn't fit him

What @Evyatarolami6 said.

Well, A to D certainly.

Because he exist

At 12 years old, in 6th grade, I eagerly turned on the TV the first Saturday morning of the new season and found Scooby Doo changed so much for the worse! There was some change in style which I noticed but accepted, and there was this awful, stupid, annoying character with a horrible voice and this stupid way of yelling "puppy power!" I just couldn't watch it.

It jarred so terribly with the old Scooby Doo, it wasn't funny, and it actually shocked me into looking closely at all the TV I watched. I woke up to the fact that almost all of it was incredibly stupid. I felt like the people writing the shows must have thought we (the tv audience) were a bunch of idiots.

At first it was just the cartoons I noticed. And if you interview professional animators who've been around since the 60's they'll tell you how awful cartoons were on network television in the 80's. They were so terrible that a 12 year old who'd never questioned anything about popular culture noticed it and stopped watching cartoons. But Scrappy had set off a chain reaction in me.

Within the year I found I couldn't stand watching TV at all except for a very few shows. It felt like I was hearing people (metaphorically) saying "wow, how stupid these viewers are" every time another idiotic beer commercial came on or another host/hostess or news announcer made another stupid joke or another horrible cartoon- like Strawberry Shortcake for example - came on. All I could bother with was anime, like Starblazers and Battle of the Planets. Also dopey, but not so outright condescending as US cartoons at the time.

That year I discovered the joy of reading. I became the biggest reader in Junior High School. Science fiction, then fantasy, then adventure and detective novels. I never looked back until I was in my mid 30s. I guess a bunch of people who liked cartoons from my age group went into animation.

Now, as a middle aged person I now watch more animation than I did at 13 years old. And very little of it is as godawful as the cartoons of the 1979 season. Or any of the 1980's or '90s so far as I know.

I understand where you are coming from, but cartoons at the time hadn't really evolved into "smart" cartoons yet. Scooby doo had been initially made on a very low budget & it was just another formulaic cartoon to have a bit of fun with. Scrappy Doo was probably included to appeal more to younger audiences. This is a strategy used by many cartoons by making a little comic relief character that isn't always done well. It wasn't until the early 2000s with the various tv movies & mystery incorporated that they started to take the characters & plot more seriously. I do love the earliest series for the cheap 70s ridiculousness & ironic ack of continuity. It's like watching a cheesy movie.