It was far from the "most reliable" when this decision was made!
>"Faced with a grim record of 4 failures from just 14 Ariane 5 launches, some observers were starting to ask if Europe's 25-year-old rocket programme is more trouble than it's worth."
>"Although developed primarily for the commercial market, a new class of heavy-lifter rockets will also be used to launch the next generation of deep-space research probes, such as the planned James Webb space telescope, Southwood says."
4 failures in the first 6 years of operation, and only 1 in the following 20 years is a good track record. How were the other launch vehicles faring in 2002?
>"Faced with a grim record of 4 failures from just 14 Ariane 5 launches, some observers were starting to ask if Europe's 25-year-old rocket programme is more trouble than it's worth."
>"Although developed primarily for the commercial market, a new class of heavy-lifter rockets will also be used to launch the next generation of deep-space research probes, such as the planned James Webb space telescope, Southwood says."
https://www.nature.com/articles/420723a ("ESA weighs up future after Ariane explosion") [2002]