There is no particular reason for the exclusion of English players from the teams, if that were the case, they wouldn't have been up for auction which they clearly were. Players like Alex Hales were up for auction and weren't bid on by the franchises. There can be a number of reasons for this:

1. Balance of the side, the fact that only four foreign players are allowed in a game. (Shaun Marsh had been prolific for KXIP but he was benched for the whole season)
2. Unavailability of players. If you look at the teams, apart from big names like Malinga and Perera (big names in T20 atleast), there wasn't much Sri Lankan participation either, this was because of the recently concluded Sri Lanka-England ODI series.

In a total of 8 teams, 4 foreign players. You have 32 international playing spots. Countries vying for these spots are 8 (excluding India and Zimbabwe and assuming only players from the top 10 ICC rankings are taken in the auction, ten Doeschate is an exception)

8 countries, 32 spots, Australia have the Big Bash League which brings out T20 talent, similarly West Indies does. There are numerous of examples of big names originating from these tournaments and making it big, hardly any such examples from England.

The Indian franchises are more willing to spend their money on current International players and those whose performance they see in local T20 leagues.

As for KP, he is recently 'retired' and is a huge name with a reputation of clearing the boundary, hence, he was picked up. Many other big names like Jayawardene and Sanga weren't picked up just like Hales.

I don't see any official or discriminatory reason of any sort if that was a part of your query.

There is no particular reason for the exclusion of English players from the teams; if that were the case, they wouldn't have been up for auction which they clearly were. Players like Alex Hales were up for auction and weren't bid on by the franchises. There can be a number of reasons for this:

1. Balance of the side, the fact that only four foreign players are allowed in a game. (Shaun Marsh had been prolific for KXIP but he was benched for the whole season)
2. Unavailability of players. If you look at the teams, apart from big names like Malinga and Perera (big names in T20 atleast), there wasn't much Sri Lankan participation either. This was because of the recently concluded Sri Lanka-England ODI series.

In a total of 8 teams, with 4 foreign players each, you have 32 international playing spots. Countries vying for these spots are 8 (excluding India and Zimbabwe and assuming only players from the top 10 ICC rankings are taken in the auction; Ryan ten Doeschate is an exception).

8 countries, 32 spots. Australia have the Big Bash League which brings out T20 talent, and similarly West Indies does. There are numerous of examples of big names originating from these tournaments and making it big, but hardly any such examples from England.

The Indian franchises are more willing to spend their money on current International players and those whose performance they see in local T20 leagues.

As for KP, he is recently 'retired' and is a huge name with a reputation of clearing the boundary, hence, he was picked up. 
Many other big names like Jayawardene and Sanga weren't picked up just like Hales.

I don't see any official or discriminatory reason of any sort if that was a part of your query.