I think there are many reasons, but the absolute leading cause in MY studio is
lack of regular practice at home - stemming from a lack of parental support and involvement, as simple as that sounds.
There are many reasons WHY there is a lack of parental support - and I will say that the culture here (southern CA, usa), does not place formal music study high on the list of priorities - we learn this from where our government chooses to spend the money, or cut it out, the ridiculously high price of concert tickets making it prohibitive for most people, the pop-music culture - etc.
But, again - in MY studio - I also blame myself to a degree. Firstly - when I first started, I was willing to take *anyone* who'd be willing to write me a check. Even somehow rationalizing a way to accept a student who's parents flat-out told me piano lessons would take a back seat to the other activities (sports mostly) in their life (when it should be as important as doing their math or spelling homework). I did "require" practice in my policy - but it wasn't *strictly* enforced in reality... and I've re-written my policy since the first couple of versions to emphasize parental support (in a bulleted list at the top) and practice and indicating the consequences of it doesn't happen (dimissal). (My policy reads big and mean b/c I'm essentially NOT, lol).
I have recently introduced certain tools and methods that make it much more clear HOW parents can help (such as, an assignment book - which I make myself & have spiral bound - that shows more clearly what the assignments are, and a simple "instruction" sheet on how to practice a piece, just for my students - which I *enforce* by doing the practice steps IN the lesson)... Plus, I've started talking about it more - meaning discussing it with the
parents.
ALL of the above simple things have helped.
Lots of these little things I've grabbed directly from pedagogy class at school - at about the same time my frustration with certain students not practicing week after week after week - and then *quitting* - was really getting to me. Thing is - I think *most* of my students' parents would help if they knew
how to help. SO, I'm helping them with that - and its working so far. The ones that aren't genuinely supportive, well, you can't do anything about that.
Story:
My son rides the bus with the girl across the street. I drop him at her house before I leave for school b/c I have to leave earlier than the bus comes. The little girl was my student (until last week). She smart, curious, interesting - smart enough to try to find ways around thinking - you know the type

. Every day I drop him off, her dad is drilling her with her spelling words for school. It hit me - THIS is the level of involvement kids need! HOWEVER, in their defense, I chose to accept them without even a *weighted* key keyboard (on the basis they'd purchase one later); and, I didn't require they come to the first few lessons to obvserve... They were neighbors/friends/ etc... I was desperate for cash. etc etc.
I LOVE the parents that say "I just want to see if little Johnny likes (the piano/music lessons) before I invest in a piano."
What I say to that *now* - Great idea. Sing them up for recreational *class* piano. At least they are going to have GOOD quality keyboards - hopefully they'll develop an interest and curiosity to know more. If they start playing on a toy - with the expectations that private teacher is going to have, you can guarantee they'll end up HATING it! OR, I tell them to RENT a piano.
I don't say that to be mean or insulting to them - it's just a matter of fact.
....
Hmm - as for the comparison to other instruments ... they can take the things into their rooms - so they have the private space and sense of freedom they need to practice. It's portable - cheaper to rent (i.e., to "try out").
...
Even for older kids - my intermediate level 14 YO would be doing perfectly fine - great probably - if it wasn't for the fact that she's overbooked! Is this entirely her fault? NO! When I was a kid, I wasn't allowed to do EVERY activity - I had to make decisions. (Partly on the basis of common sense, partly b/c we weren't rolling in $$$). I think requiring older kids to make decisions - what to cut out and what to keep, so they can excel at the stuff they want to do most - is just good parenting.
Anyway - that is MY data and what's been true for my students.
Forgive me for rambling...
