It's because we value a lot of Slytherin qualities and see many Slytherins as anti-Heroes, which are more interesting than Gryffindors. While Gryffindors are brave, they seem to be very wreckless, over-confident and hot-headed. We see a lot of successful gryffindors in the series, but these are either already talented, famous, or have sheer luck. Except Dumbldore's Army, we find no instance of gryffindors trying to actively shape their destiny. Most gryffindors either make decisions on the spur of a situation, or getting carried away by their feelings. They seem to lack ambition, willpower, and diligence to make long-term changes either in their lives, or for the wizarding world. Their actions are always very reactive, as opposed to pro-active.

I mean, sure Slytherins like Malfoys have a lot of connections, but Tom Riddle was born with literally nothing, and yet he climbed his way to the top, and by the time he left Hogwarts, he was a favorite of all teachers, and had acquired many honors and medals. Severus Snape, through sheer willpower, patience and courage turned out to be one of the most useful intels in the Order of Phoenix. Horace Slughorn was a warm and friendly person, and throughout his life, created a well-connected network of famous people, and lived his retirement in luxury and respected by all (all because of his talent and socializing). The Malfoys appear to be pure-blood supremacists, but they always seem to know to draw the line and their own family was always priority number one, Voldemort was always priority number two.

There are some Gryffindors with Slytherin qualities. Hermione is extremely meticulous and takes all precautions during their hunt for Hocruxes. Harry Potter is very resourceful and has learnt the art of persuading people really well. Albus Dumbledore is very patient and makes calculated risks. Sirius Black's strong desire and ambition to expose Peter Pettigrew, kept him alive in Azkaban and helped him engineer an escape. The most interesting aspects of Gryffindors happen to be their Slytherin qualities.

Of course Slytherin qualities also involve negatives, like a general apathy towards others, encouraging hierarchies and profiling as opposed to equality, and a sense of entitlement. But we can hope good Slytherins in the future can redeem those qualities.