The reason a new ISP will use VLAN10 is because it's less friction when a customer brings their old router over from a provider who uses it.

Then it's a matter of just moving the connection and you're done.

If the ISP isn't ordering tagged connections, then when the customer BYOD's then they need to talk their customer through logging into their device and moving the DHCP/PPPoE interface onto an untagged port of their CPE. That's a support call/cost.

It's a numbers game - more customers are going to have a VLAN10 connection than aren't, due to the fact all the big players with the most customers use it.

PPPoE is great for maintaining state easily (so you know when the connection is down) but it adds the overhead of the CPE having to encap/deencap every packet.  This can, depending on the CPE, make a big difference to maximum throughput a CPE can achieve.