So today I decided to download the demo for Nintendogs. Then I noticed that this demo in particular has only 10 uses as the others have 30. Why is this?

Because who would play a Nintendogs demo more than 10 times? simple logic.

I think it's because Nintendogs is the type of game where you only need one play through to decide whether you like it or not.

Don't you mean 1 time?

Well I wouldn't play through it once myself, or even bother downloading it...

This. They even give you 9 more tries.

To be honest, I haven't used any of the demos more than twice.

I have a young daughter who loves pets, and a wife who hates pets. Wife overrules daughter, so we are a pet-free home. This demo has captivated my daughter. She asks to play it every morning after getting up, laughing and giggling as she plays with her pretend pets. It's gotten to the point where my wife is suggesting that maybe we should buy a copy of the game.

If the demo was unlimited, we wouldn't buy because it's good enough for my daughter "as is". If there were fewer uses provided for the demo we wouldn't really know how much my daughter enjoys the game or whether her interest would fade quickly. I think giving 10 uses of the demo is good enough to provide a meaningful sample size while still forcing gamers to decide whether or not they want the product without having that "new game" feel wear thin.

After 10 times I'll be bored of poking dogs and cats.

It's probably because Nintendo thought giving you 30 tries raises the block number immensely.

You can play demos as many times as you wish if you don't turn it off and it only counts as one use.