I would say because it was not necessary to cast the spell non-verbally.
In this scene Snape is concentrating on casting the counter-curse to keep Harry on his broom. To maintain the counter-curse he needs to keep his eyes on Harry.
The person casting the curse (Quirell) also needs to keep his concentration on Harry:
“No, no, no. I tried to kill you. Your friend Miss Granger accidentally knocked me over as she rushed to set fire to Snape at that Quidditch match. She broke my eye contact with you. Another few seconds and I’d have got you off that broom.”
- Harry Potter and the Philosopher's Stone, Chapter 17
credit to Janus Bahs Jacquet for the quote in the comments
(My emphasis). Quirrell's curse loses its effectiveness when his concentration is lost.
We might infer that it is therefore very unlikely that Snape would have been spotted by Quirrell casting the counter-curse - Quirrell's eyes would have been on Harry. Snape may well have made this calculation himself.
Also, with regards to anyone else spotting Snape... as far as Snape is concerned, he is protecting Harry - he has nothing to hide, so why hide that he is casting the counter-curse?
There is no strategic advantage to be had in Snape hiding that he is casting the counter-curse, and therefore it is more trouble than it is worth to cast the spell non-verbally.