Why do people think MW2 is better than black ops?

It is better.
Despite both games having their own individual flaws, MW2 is still a more successfully executed FPS.
The precision of guns, equipment and even the sound effects they generate are more realistic and streamlined in MW2.
Black ops graphics are on a way lower level, the sound effects are nowhere near as crisp and the general gameplay is seemingly arcadish compared to MW2.
Also in black ops, there have been many changes (additions and removals) that have affected the ultimate gaming experience of it. Some improvements include: two player multiplayer, combat training, additional customization.. However despite these additions being a move forward they are meaningless if the online multiplayer gameplay is inferior to previous COD's.
The major problems with Black Ops all seem to originate with Treyarch over thinking things.
A major issue is with the new in game currency system. COD points are acquired through player matches and wager matches. These points are an interesting addition to the game but they become useless because as soon as you have purchased a gun you automatically get access to every attachment and camo (except gold). Meaning, unlike in MW2 where you actually had to earn the special attachments, you can straight away buy the best attachment for that gun without even firing it once.
Finally the most tarnishing fact about COD - Black Ops is the fact that they spent so much time trying to balance perks and killstreaks that they completely unbalanced the weapons. Shoguns are incredibly short range, okay fine, but they are also incredibly prone to hit markers.. umm i think at short range you'd die one shot from a shot gun.. Snipers are also now inadequately slow, inaccurate and stupid. In fact if your not using one of the 20 or so SMGs and ARs you'll find yourself at a severe disadvantage playing online. MW2 was great in the fact that most maps were suited to every weapon type and it was up to the player to decide on their class layout and gaming style. Now in Black Ops you are refined to conform to the general rush and spray tactics forced upon you by the new map designs and weapons.
These flaws may seem minor but the fact is they ultimately affect the overall gameplay and in turn your gaming experience. I can speak from first hand gaming exp and all i know is that i still play MW2 and i have owned it since lauch night. Black Ops on the other hand ive had for a couple of months and already find myself sitting it down somewhere hidden amongst my other xbox 360 games.
Sorry i got a bit carried away,
I'm very passionate about MW2, bring on MW3!

Mostly because they have status and rank from playing Call of Duty Modern Warfare 2 for so long and when call of Duty Modern Warfare 3 comes out they will be the same people who will claim MW2 is better even if they never even purchase the game