Why? What's the disadvantage for them?

Just the installation cost?

Mostly they don't want to mingle with the hoi polloi.

Atherton (not Belmont – all those shitty little peninsula towns blend together in my mind) sued because they didn't want trees removed to make way for overhead wires (the claim was that "cleaner" diesel would be better than electric)[0]. Then they sued because the poles were going to be 10 ft higher than promised[1]. Another Atherton resident[2] sued because he wanted Union Pacific to explicitly approve everything beforehand. More dreck[3].

Caltrain's getting sued left and right over the electrification, starved for money by BART, and generally ignored by politicians. If you're ever curious about why Bay Area transit is as bad as it is, Caltrain is a great case study.

0: https://en.wikipedia.org/wiki/Electrification_of_Caltrain#La...

1: https://www.almanacnews.com/news/2017/12/21/atherton-battlin...

2: http://www.cahsrblog.com/2009/08/another-peninsula-nimby-law...

3: https://caltrain-hsr.blogspot.com/2009/08/atherton-et-al-law...

To make matters even more frustrating, Atherton starved their Caltrain station to death:

https://www.almanacnews.com/news/2020/01/17/atherton-signs-o...

Belmont at least has some density and Main Street. Atherton’s density is 1/4 of Belmont’s and it occupied the very center of SV.

Atherton 1,438.97/sq mi (555.53/km2)

Belmont 5,843.98/sq mi (2,256.45/km2)

San Mateo 8,570.40/sq mi (3,309.14/km2)

Not hard to see what their real intent was.

When some have the freedom of protecting their private land, others end up with the freedom of riding diesel only, underfunded trains. The liberty of some seems greater than others’.

I wonder a bit though as advocating for diesels service seems a bit like they're shooting themselves in the foot, at least in terms of air quality.