There were several reasons for the lack of traffic to and from London via the Thames & Severn.
The Kennet & Avon Canal which joined the Thames at Reading, opened in 1810, providing a route from Bristol which avoided the difficult upper reaches of the River Thames, west of Oxford. Many cargoes were carried in larger boats sailing around the coast, particularly after war with France ended in 1815.
Customers were also put off by the disruptions and delays. Between 1827 and 1830, the Thames & Severn was closed for an average of nearly seven weeks each year because of ice in winter, water-shortages in summer and frequent repairs.