Q: Why do you cap Hit and hard-cap Expertise?
Capping both of these guarantees Holy Power (HP) generation every time a generating ability is used. This leads to a higher Shield of the Righteous (SotR) uptime, resulting in a much smoother damage intake. Aside from eliminating as much RNG as possible, the extra damage is a big plus. There is no completely right or wrong way to gear your Protection Paladin (more below).
Q: Okay, so what about the haste stacking, that’s a bit strange?
The changes to Sanctity of Battle (making it available to Protection) with the launch of MoP has turned Haste into a very attractive tanking stat. The CD and GCD reduction on all of our abilities leads to a much higher HP generation leading to a higher SotR uptime (or WoG use), meaning less damage taken. The other big bonus is the added damage dealt that comes with this build, which helps quite a bit.
A final note about Haste is the exceptional healing it will provide. Since you attack more, you will heal more often with Seal of Insight, and also output more raid healing if using Glyph of Battle Healer. A further HUGE consideration is the benefit to Sacred Shield. More Haste means more Sacred Shield ticks, which absorb more of any damage type. This is a massive consideration towards Haste that has also been shown mathematically.
Q: Oh this all makes sense! Are there any other viable builds out there?
For sure! A very competitive build to the hit/exp cap + haste stacking one is to stack mastery instead of haste. In terms of damage reduction the builds perform at essentially the same level (mathematically). However, again Haste has the big bonus of a nice chunk of increased DPS. Some things are situational however, and if you are pushing progression heavily sometimes Mastery might be the smarter choice for a particular fight, that is up to you as every fight is different and you do what you can to win.
Other viable builds include a classic avoidance build, stacking as much dodge and parry as possible (no hit/exp), or stacking Mastery (again no hit/exp). These builds are unfortunately highly susceptible to nasty spike damage, therefore making them not preferable for myself.
It is also is common for some people to soft-cap expertise (7.5%), thus eliminating dodge, and then stacking Haste or Mastery. The difference between soft and hard-cap is minimal, and hard-cap still pulls ahead slightly due to the reasons mentioned in the first response.
Although there are builds in-between and this particular answer can carry on, these are the among the major ones listed above.