Probably because weed growing developed independently from other agricultural industries for several decades, the term strain was coined when it still was illegalized everywhere.

To sound more sciency/marketing because in plants the more normal term is "line". Tho strain and line are usually interchangeable.

In plants where the main propagation method at the production-scale is artificial cloning of highly-heterozigous specific individuals, for example forestry, flowers and cannabis, to describe different germplasms you use "line" or "strain", instead of "cultivars" which are germplasms propagated via its normal reproduction method while maintaining its genetic qualities like soybeans and wheat.

[Soybeans and wheat are naturally autogamous and in result high/complete homozigous so its normal reproduction is basically cloning but effortlessly.

In contrast maize is naturally alogamous and have inbreeding depression so you need to put effort to make it highly homozigous to create a genetically stable line which are only useful in breeding/hybrid-lines-production and arent actually used at the harvest-production scale]

BUTT I've never seen people in forestry talk about strains because they use the term "line" instead (a usual term in plants and animals) and in pines and eucaliptus you don't need sciency jargon for marketing purposes.