I just want some help justifying how a 555 timer could be a better choice rather than saying it just easier.

If you put either system (555 and uC) inside a box, could you tell from the outside which one was inside?

- both sense water the same

- do they use the same current? Probably damn close

- how much did they cost? Poking around digikey I see I can get a 555 for 43 cents and I can get a uC for 49 cents. So the other parts may need to be costed to see if there is a clear winner there. Assembly costs may also need to be factored in: it is a very close race.

BTW, when I made a water sensor decades ago I used a CMOS 4011 quad NAND gate.

 

The application is a liquid detector, please see the image.

I could have used a MCU but I found it easier using a 555 timer. I just want some help justifying how a 555 timer could be a better choice rather than saying it just easier.

First off, being "easier" is a perfectly valid justification. What you should do to back it up is describe WHY it is "easier", which will also force you to flesh out the notion of just what "easier" means to begin with.