I think one reason is taste (not culinary, aesthetics). And, as they say, an argument over taste cannot be had. But taste is a delicate creature. Often fleeing. Ever evolving. Similar to fashion in a big way. Culture and all that occurs in a society and the societal history creates current taste. It is inherited from our parents and shaped by our friends, what we read, watch, contemplate, what we are told is good or bad, beautiful or ugly, etc. It involves lots of prejudice too. It often flows from prejudice that common, lowly and plain things are overlooked, thought of as boring and few are those who see beauty in modest places. For one of those folks, there are 1000 of us who, like a fish, are lured by something flashy, cool, exotic, desirable by many. Let it be possessions, people, pets...
I think another reason is along the same lines that push us to get new fish and new, bigger tanks. We get bored. We need to get re-excited. It's a temporary fix with no end in sight. I am talking average statistics. Most certainly there are exemptions to any rule. It may be argued that this reason is too a part of the multifaceted taste - a whimsical thingy that may be a siren.