because BrE AmE bi ˈkɒz bə-, -ˈkəz, -kəz, § -ˈkɔːz, § -ˈkɒs, § -ˈkɔːs AmE -ˈkʌz -ˈkɔːz, -ˈkɑːz, -ˈkəz, -kəz —Many speakers use bi kəz (or bə kəz ) as the weak form, bi ˈkɒz AmE \ -ˈkɔːz (etc.) as the strong form. Some, though, also use an irregular strong form bi ˈkəz, bə ˈkəz . There are also casual variants kɒz, kəz (etc.) —see cos.
— Preference poll, American English: -ˈkʌz 57%, -ˈkɔːz or -ˈkɑːz 41%, -ˈkɔːs or -ˈkɑːs 2%.