This is a question I received from Quora.

The answer is straight-forward: rapid depreciation and loss of value.

However, there are a few subtleties that need to be taken into account.
This might be of interest to some who programmed their own Cryptocurrency and hope by holding a majority of the coins stand a chance of being “wealthy”.

Sources:

Mining cap: There is a total of 21 million Bitcoins that can be mined.
As the hyperlink suggests, the last bitcoin will be mined somewhere around 2140.

Lost Coins: Bitcoins (or other Cryptocurrency) can be lost and not be recovered. The method of losing it can be through online wallets crashing, digital ledgers breaking down (or being lost), website crashes, hardware wallets breaking.

Original Quora article.