Bristol Mayor Marvin Rees believes the city did not have the time to approach other companies over the development of Temple Island.
In the wake of the publishing of a proposed deal for the site with L&G, Mr Rees said opening talks with other groups would four to five years.
The city council's cabinet members will make a decision on the proposed agreement next week.
Working with L&G, the council would invest £32m for 500 new homes, a 250-room hotel and a conference centre at the site by 2025.
The location was once the proposed home for an entertainment arena.
Mr Rees spoke on Emma Britton's BBC Radio Bristol show about the deal this morning.
When asked why other companies were not approached, he said: "This is about pace of delivery, so our point is can we get pace of delivery, with Bristol not standing on stagnant sites?
"If we can get the pace of delivery with L&G already here, but knowing it is already held up to the rigour of government standards on green book validation, red book validation and legal tests and financial tests then let's do that.
"It [approaching other groups] will take another four to five years and getting nothing done, increasing costs and losing opportunities to the city."
Asked why Bristol needs the development, Mr Rees said the city had fallen behind others in the country for facilities such as conference centres.
He said: "Bristol has not been a city that's got stuff built in the past.
"If we are going to build up Bristol to deal with our growing population and climate emergency we are going to work with big companies we need that scale and volume of development.
"We've looked historically at Cardiff, Newcastle, Nottingham, Birmingham and Manchester with real envy and now we are in that league of getting things done."
