Because it’s hard
A quarter mile diversion can't take that long to dig. The whole canal is 120 miles, so an extra quarter mile they should be able to dig in a few days I would think?
(If you think it's that easy, pop over and ask if you can have a go on the JCB)
Uh, what? Why would the length of the canal as a whole tell you anything about how long digging a quarter-mile section would take? If the canal as a whole was longer or shorter, would that somehow change your estimate?
Once-per-century investment for France. It was, like the Eurotunnel, nominally privately funded: https://en.wikipedia.org/wiki/Suez_Canal_Company but with significant political cover.
If you had to estimate the volume of earth in that 'quarter mile', and then estimate the size of an excavator bucket, and the capacity of a dump truck, and the availability of all those things at any given time (even if you ship them in), you'd soon realize that just digging out that earth will take more time.
On top of that, there's going to be FLOW in the canal - what will your diversion to for erosion, pressure on the locks, etc.
You risk damaging the canal itself to deal with a temporary problem if you think you can just 'dig around it' in 5 days. My bet is you've never seen how long it takes to plan something with this much risk involved.
This isn't software, you aren't going to Agile it away if you screw it up.
- launch minecraft in creative mode and make 40x40 pad of steel blocks, 1 block high. You should do it in a few minutes.
- now do a 40x40x40 cube. Good luck for the rest of your day.