The most common reason is growth. An account that suited a beginner at £0 often outgrows the standard pricing tier, and a raw-spread account with commission elsewhere becomes cheaper once monthly volume rises. Cost structure, not cost headline, is what changes.
The second is product range. Trade Nation runs TN Trader, MT4, TradingView, which suits some workflows and blocks others outright. Traders who decide to automate, to use depth of market, or to hold shares in an ISA alongside leveraged positions frequently need a firm built for that from the start.
The third is service. Execution in fast markets, withdrawal turnaround and the quality of the answer you get when something goes wrong are the parts of a broker that never appear in a comparison table, and they are the parts people actually leave over.