Question

why b comes after a?​

Answer 1

because a is before b

Answer 2

We don’t really know whether there was any logic behind it. The Wikipedia article on suggests that the earliest alphabet came into use between 2000BC and 1800BC, and it isn’t known whether there was an associated standard order. The earliest evidence of alphabetical order comes from (a northwest Semitic language) some time around 1300BC. Ugaritic had not one but two orders: Northern Semitic Order (from which the Latin alphabet is derived) and Southern Semitic Order. There’s no reason to think either of these orders was anything but arbitrary at the time of its origin, though.