MetaTrader is not the only platform that can run a trading program, but it is where almost all retail Expert Advisors are, and understanding why helps when a strategy or a broker is on something else.
cTrader
cTrader is a platform offered by a smaller set of brokers, popular for its interface and its depth-of-market display. Its automation is called cTrader Automate, and its programs, cBots, are written in C#. It has a backtester and a cloud service for running bots. A cBot cannot run on MetaTrader and an EA cannot run on cTrader; a strategy moving between them has to be rewritten.
TradingView
TradingView is a charting and community platform rather than a broker terminal. Strategies are written in Pine Script, and they can be backtested and used to generate alerts. TradingView does not itself execute trades on most brokers; automation is achieved by sending alerts to third-party services or webhooks that place orders, which adds a link in the chain and a delay. It is a good place to prototype an idea and a poor place to run one unattended.
Others
NinjaTrader, MultiCharts and a number of proprietary broker platforms offer automation, largely for futures and equities, with their own languages. Some brokers expose an API for programs written from scratch. Each is a separate ecosystem.
Why MetaTrader dominates
Broker availability, since most retail forex and CFD brokers offer MT5 and many offer only that. The MQL5 Market and Signals, which give developers a distribution and licensing system and traders a place to find them. The verification services, which connect to MetaTrader accounts most easily. And the sheer number of existing EAs and developers. A strategy on MT5 can be sold, verified and run at almost any broker; a strategy on a less common platform cannot.
A trader builds a promising idea as a Pine Script on TradingView and wants to run it live. The options are: connect TradingView alerts to a webhook service and a broker API, with a delay and a monthly fee; rewrite it as a cBot for a cTrader broker; or rewrite it as an MQL5 EA and run it on MT5 at any broker, verify it on any service and, if it works, sell it. The third is more work up front and less work forever after.
An EA is an MT5 program and runs nowhere else. If a broker offers only cTrader or a proprietary platform, that broker cannot run it. When comparing automated products across platforms, the record and the rules matter as they do anywhere, and the platform's own verification arrangements matter too; MetaTrader's read-only investor password is the simplest and most widely supported form there is.