tl/dr; ASN's are highly visible to network engineers, it's your identiy as a network. Lower/shorter numbers are "better", at least to some people.
All of the 4 digit ASN's were originally allocated in the 90's, during the initial ISP/dot com boom and are often viewed as a sign of being an established player, or just "cool". Almost all the major ISP's use 3 or 4-digit ASN's: 3356 = Level3/Centurylink/Lumen, 701 = Verizon, 174 = Cogent, 1299 = Telia (now Arelion), 2914 = NTT, 6453 = TATA, etc. Even Netflix snagged 2906 when ARIN suddenly recycled a bunch of 4-digit ASN's in 2009.
If you are an ISP, every one of your BGP customers and peers has to configure a BGP session with your ASN so it has maximal visibility in that use case. Even if you are just peering on public IX's, every peer will see your ASN when they configure the session or do "show bgp sum".
These days there is a similar effect though less dramatic for 5-digit ASN's, and that is reflected in the lower asking prices.
What surprises me is how fast the 5-digit ASN's were selling (look at recent sales). In the ARIN region you can get a 5-digit ASN just for asking when you apply, otherwise they assign a 6-digit by default most of the time. The takeaway from this is that people are paying for a definite, specific number that they like rather than take their chances with a random assignment from ARIN.