Why does Blood Moon remove abilities?
This appears to be rule 305.7, which I got from: Gamepedia.

305.7. If an effect sets a land’s subtype to one or more of the basic land types, the land no longer has its old land type. It loses all
abilities generated from its rules text, its old land types, and any
copy effects affecting that land, and it gains the appropriate mana
ability for each new basic land type. Note that this doesn’t remove
any abilities that were granted to the land by other effects. Setting
a land’s subtype doesn’t add or remove any card types (such as
creature) or supertypes (such as basic, legendary, and snow) the land
may have. If a land gains one or more land types in addition to its
own, it keeps its land types and rules text, and it gains the new land
types and mana abilities.

Note, the old wording, on the original card in The Dark expansion uses &quot;basic mountain.&quot;  My guess is that this rule was added after.