The reason is simple.

When capitals were introduced they could be built in and enter high sec. I believe that cyno's could also be lit in high sec. 100% of the player base could use them, as is fair since 100% of the player base pay for the development of EVE.

Now unfortunately, there is a very tiny whiny portion of the player base that feel entitled to 100% of the development of EVE while denying most of the development to the majority of the player base. Lets call this tiny portion Poop Socking Nerds or PSN's. The latter group we'll call the Ignored Subscribers of EvE or ISE's.

So when caps were released everything was fair, however the PSN's cried and whined and grovelled to CCP to make caps only available to them, and to remove caps from use by the ISE's. CCP being best mates with a lot of the PSN's (due to the way the company seems to mix inappropriately with PSN's - Case in Point Link) decided that was a good idea.

Some of the arguments were along the lines of Caps can do missions! Caps can insta-shot ships. Caps crash into stations (lore BS) but not really were worthy of the bytes they wasted on the interwebs. They were **** poor arguments made for the xpress purpose of removing content paid for by all and giving it to a very small segment of CCP's best mates - the PSN's or null sec crowd.

The simple fact is there is absolutely no reason for caps not to be able to enter high sec. But like everything in EVE if the PSN want something exclusively they're conveniently flown to Iceland every once in a while, likely go boozing with the devs after having their private chats and get what they want.

If CCP was to be transparent, fair and give everyone equal content for their subscriptions they would do away with their arbitrary prejudicial limitations, do away with their rigged system of "electing" CSM's (I'd be happy to volunteer to be a CSM, I have been playing since 2003, have a good understanding of the game, have alternate views about what should be happening, but currently while I could probably get 1k votes, the big alliances can get up to 37k x 3 (with alts)). So what you have is a CSM that is entirely for PSN's and nobody else.

In short if you want to know why anything in EVE is the way it is, look to the CSM, look to the alliance influence on CPP and look to the vote rigging whenever elections come around.

The best thing to do is ignore the CCP / Alliance Cronism and try to enjoy the game as best you can. Things won't change.