More precisely, a sunk cost thing.

The costs of widening bridges, viaducts, structures, and tunnels for the other railways would be immense. To narrow the Great Western only required resetting one of the two running rails. Founder I.K. Brunel had been shortsighted in his belief that the Great Western would receive all the traffic of its territory, with no need to connect to the other lines of Britain. Lengthy debate in Parliament and various commissions finally concluded that a single gauge was necessary, and that "standard" gauge was the one. There were 2000 miles of standard gauge track; only 300 miles of the broad gauge.

Though not a big factor in Britain, it's worth noting that engineering costs in mountainous areas decline significantly for a narrower gauge. The British built many colonial railways to 3-foot-6 gauge, and other colonial powers built metre-gauge lines throughout Southeast Asia for that reason.

Standard gauge won out for three main reasons :

Construction costs for track and for rolling stock was rather less than for the broad gauge, whereas the carrying capacity was much the same. For all the large track width the loading gauge ( I e the size of the wagons and carriages ) was only slightly larger. Brunel was concerned, as were other engineers at the time, about stability of railway vehicles not about building larger trains. Therefore the 7ft gauge placed the wheels at a width to prevent toppling over. It became apparent quite quickly that the stability of standard gauge trains was not a huge problem. Indeed quite obviously much larger trains now run on the standard gauge.

The interchange costs between standard and broad gauge were huge, and became increasingly unpopular. As new lines were built that encroached into great western territory the great western began to lose market share to competing standard gauge railways.

The government having adopted a laissez faire attitude to the early railways suddenly became very aware of the advantages of being able to move troops around the country at speed. The additional delay of transfer between broad gauge and standard gauge was problematic. Although the government in this era would never tell a private company what to do, the report of the parliamentary inquiry into railway gauges emphasised the benefits of a single gauge and in practice meant there was very little expansion of the broad gauge afterwards.

It is a fascinating period of railway history and if you are interested I would recommend that you seek out pamphlets published by proponents of both the broad and standard gauge (which at the risk of the 20 year rule resemble mac vs windows !) . The National Railway Museum in York has a selection in its archives. The report of the Commission is available online via the truly excellent Railways Archive.