Simple answer

Because most of it is staked

Slightly longer answer

Most of the people that hold ADA, even if it is a small amount, are holding it in either Yoroi or Daedalus wallet.

Because Not your keys, not your Crypto

And while sitting there it is not doing anything, so most people think why not just stake them for some returns.

Since it is just a few click and super easy to do, many end up doing it.

So you have a lot of users stacking their ADA on official apps.

However there are other wallets that are multicoin wallets which also give option to stack. Again with just a few easy clicks.

In a way that is the most logical thing to do, if you have an option to earn interest on your holdings why not do that!

However, this prevents ADA from being sold.

Don't get me wrong, you can easily sell your ADA if you want to, since there is no lock up period.

But, if you are already making say 5% on your stake, would you want to move your coins out of your wallet and sell for fiat or stablecoin.

Furthermore, since Cardano has been rising in price for some time now, you have an expectation that it can go further; Maybe even take over Ethereum.

However, if I had those coin on an exchange I might consider selling them high and buying them back when the price dips.

Perfect incentive

Since I also have my few dollar worth of ADA on the official wallet I am reluctant to transfer them to an exchange and taking part in the speculation. I would incur fees and since there is a chance that if I sell my ADA thinking it is the top I might lose out on the higher price waiting for the dip that may not come.

This creates a perfect loop for people to hold ADA, which feeds on itself to grows.

It is quite brilliant actually.

Conclusion

Since this is a short topic I didn't want to put in irrelevant and off topic information just for the sake of length.

Therefore, for now I just leave you with this closing thought,

A lot of people think that Cardano is overhype and just all talk but the very same people will also be holding ADA in their portfolios.

Cardano could replace Ethereum or it could fail miserably, only time can tell.

But right now it is the only Smart blockchain that could be a serious contender to Ethereum.

Just don't expect it to be anything like $100 or more because of something called Tokenomics

At best, if Cardano becomes larger than Ethereum (a very hard task indeed) you can expect the price to be around $10-$12

However since the Smart contract are expect to go live on the mainnet soon and we could see Cardano move to its Phase 3 called "Goguen", this could potentially increase the use cases of the Cardano blockchain and we could see a lot of Ethereum functionalities like NFTs, STOs and dApps shifting to Cardano from various different blockchains.

We already have token on Cardano and they have been a lot of development going on it.

Only time will tell if Cardano will be just another Ethereum wannabe or will it be the mythical Ethereum killer.

This is one of the reason I convert some of the BCH I received here to Cardano (ADA)