If we take my area around Hertfordshire, the answer I suspect is quite simple. Most people simply don’t want to use buses, and have access to superior alternatives, which in most cases will be their car. So the only viable bus services will be those in or serving larger towns, and even then only a basic service.
If you take Hertfordshire, there are buses on the key corridors in to the evening - up to circa 11pm
So going West to East:
500 Hemel - Aylesbury runs hourly into the evenings.
321 Watford - Luton runs hourly into the evenings.
301 St Albans - Stevenage runs hourly into the evenings
302 St Albans - Hemel runs hourly into the evenings
101 Luton - Stevenage have departures from Luton at 20.40 and 22.40 and Stevenage at 19.45 and 21.45
724 Harlow - Heathrow - last journey from Harlow at 20.30 going through Herts after 9pm - there are also departures from Harlow at 01.10, 03.00, 03.45 and 04.30
510 Harlow - Bishops Stortford - Stansted runs hourly in the evenings.
Most Stevenage town services run hourly up to 11pm.
And in the rural areas, it's just been announced that the DRT service HertsLynx will be running late on Friday and Saturday nights.
You also have rail services linking the following:
Rickmansworth - Watford (Met)
Watford - Hemel - Berkhamsted (LNW)
Borehamwood - Radlett - St Albans - Harpenden (Thameslink)
Potters Bar - Hatfield - Welwyn Gdn City - Knebworth - Stevenage - Hitchin - Letchworth - Baldock - Royston (Thameslink / GN)
Cuffley - Hertford - Stevenage (Thameslink / GN)
Bishops Stortford - Sawbridgeworth - Harlow (GA)
Hertford - Ware - Broxbourne - Cheshunt (GA).
It's a pretty good service across train and bus into the evening unless you happen to be the OP who has some particularly obscure and specific need which no reasonable public transport system can meet.