I was looking at ifr low enroute chart and just figured out that KSEA and KLAX are depicted as class D. Aren't they Class B airports? why do they have D mark?
Your question is interesting but the answer is not fully obvious without a bit of research. In looking at the KSEA IFR Enroute chart (L01 IFR enroute chart -third image below) it does indeed show that a portion of the the Seattle International Airport is within Class D airspace. The second image below shows an excerpt from FAA Order 7400.11E describing the Class D airspace as it relates to KSEA. The third image is from the L01 IFR Enroute Chart for Seattle.
So, although some general information relating to the OP's question may be addressed by looking at the link shown above by @randomhead as it applies to LAX, the OP is asking a reasonable question which is fundamentally different (from the question addressed in the link mentioned above) and probably should be left open.
Further, it is a fairly unusual circumstance to have Class D areas attached to the primary airport upon which the Class B airspace is predicated, so a discussion relating to identifying the affected areas that are Class D versus Class B may be very informative.
Likely, the Seattle Class D airspace (dimensions shown above), is to handle go-arounds, (that turn and enter a downwind to return to the airport as opposed to returning to the TRACON to be vectored out for another full [longer] approach), helicopter traffic, traffic watch aircraft, etc. (being on or staying on the Control Tower's frequency, for example) .
If you do the research for KLAX, it's the same principle (but different dimensions for the Class D).