Not sure whether this is the usual BA baiting, but as an older new guy here are some reasons for joining (all been said before I'm sure).
1. The ability to pick and alter your roster even as a junior bod (no 'decade for a weekend off' in my experience).
2. Security.
3. Choice to change fleets.
4. Decent pay (certainly after a few years).
5. Decent hotels (for the most part).
6. Massive network.
7. Worldwide staff travel.
8. Very low cockpit-gradient culture (hence long-term RHS isn't a huge problem, not for me anyway as an ex-Capt).
Now, there's no doubt BA isn't the Mecca it once was (it has its downsides too) and I'm sure other airlines offer some, if not all, of the above but these were pretty good reasons for me and there are definitely no shortage of applicants when the doors open which is a pretty good indicator.