Because it is one of the only Celtic languages left. They used to be spoken all across Europe, but over time they were cornered into pockets in France and the British Isles. Many Celtic languages have already gone extinct (Cornish, Manx...) and there are only about 300,000 speakers of Irish Gaelic left and English is still creeping into those few Gaelic-speaking areas. Is it so wrong for them to want to keep the tongue of their ancestors alive, to preserve that little bit of their culture from dying forever? Note that most Irish Gaelic speakers are bilingual in English. But this problem is much greater than Irish Gaelic. Languages go extinct all the time, mostly those of conquered peoples who adopt another language to "fit in" with their society. If this trend continues, everyone on Earth will be speaking only a couple dozen languages, or maybe everybody will speak English and all foreign languages, all the foreign literature will be lost to the new generations.
If some Irish people want to use the tongue of their ancestors amongst themselves, I don't see how that is anybody else's business. It is the Irish language (the first official language of Ireland in fact), and although Irish English is more widespread, you know the Irish. We don't go down without a fight.
If over time, for example, Spanish started becoming the dominant language in parts of the United States and English speakers started becoming a minority, there would be no reason why they ought to abandon their native tongue in favor of the more popular tongue, although it may be still beneficial to learn Spanish. Fact is, English wouldn't exist if everybody followed your logic. There were several centuries when French was the sole official language of all England.
