The original manuscript is not extant here. The verb were (or was) would have occurred at the end of a line in 𝓞, so it is quite possible that 𝓞 actually read was but was miscopied by Oliver Cowdery as were. In the early 1900s, the LDS text adopted was in place of were. The 1906 largeprint edition was the first one to replace the were with was, an emendation followed in the third printing of the 1905 Chicago edition (in 1907) and in the 1911 large-print Chicago edition (and in all subsequent LDS editions). The 1953 RLDS edition also made this change, probably independently of the modern LDS text.
Elsewhere in the text we have two other occurrences of “because it was”, but none of “because it were”:
Although the evidence is meager, these two examples support emending “because it were easy” in Alma 53:5 to “because it was easy”.
The Book of Mormon text, of course, has many examples of it were as a subjunctive form in conditional statements, such as “save it were”, “if it were”, “as it were”, “except it were”, and “whether it were”. Even so, there are some firm instances of it were in the earliest text that would normally read it was in standard English; some of these instances of it were have been changed to it was, but not all (each of the textually invariant cases is marked below with an asterisk):
These instances of it were are equally as strange as the one in Alma 53:5, yet only half of them have been changed to it was (namely, Alma 55:23 when Oliver Cowdery copied from 𝓞 into 𝓟, and Mormon 2:16 and Ether 15:14 in the editing for the 1837 edition). The critical text will therefore restore the original use of it were here in Alma 53:5 since it appears to be intended.
Summary: Restore in Alma 53:5 the occurrence of it were in “because it were easy”, the earliest reading; despite its nonstandard form, the original text has other instances of it were in nonconditional clauses that have never been edited to it was.